In a scene at the exterior of the Chapel, as church bells ring, the Morgans exit following a grim marriage ceremony - their daughter has entered into a loveless contract with the son of the wealthy mine-owner. The unhappy, unsmiling, glassy-eyed face of Angharad emerges as she proceeds down the steps with Iestyn to an open carriage driven by a coachman. [Bronwyn's earlier celebratory marriage to Ivor is starkly paralleled.] Her white veil billows high behind her in the wind. Music is finally forced from the silent witnesses when Mr. Morgan asks Dai Bando: "Is there to be no singing for my daughter's wedding?" As the carriage pulls away, the silhouette of the lonely preacher, who has emerged from the back of the chapel and stands under a tree in a cemetery, watches from afar.

In the Morgan kitchen table one night, Mr. Gruffydd mentors Huw in his studies with "a problem for the mind" in preparation for his entrance examination into the National School in the next month: "The bath tub holds one hundred gallons. 'A' fills it at the rate of twenty gallons a minute. And 'B' at the rate of ten gallons a minute...'C' is a hole that empties it at the rate of five gallons a minute. How long to fill the tub?" Playfully, Beth doesn't understand the illogical problem of a bathtub with holes in it.

Huw's one departure from the village occurs when he walks to the National School over the mountain in a neighboring valley for his first day of school - it is a very traumatic experience:

Who is there that cannot look back and remember his first day at a new school? To go alone the long walk over the hills to the next valley. The first of my family to have the privilege of attending a National School.

As Huw warily enters the classroom of clean, well-dressed boys and girls, all eyes turn toward him. And then, the arrogant, unpleasant, sadistic teacher Mr. Jonas (Morton Lowery) at the front of the room treats him with sneering ridicule:

So you're the new boy...You're late!...What a dirty little sweep it is!...A little genius from the coal pits. And they expect me to make a scholar of it. All right, come in. Were you brought up in stables? Well, shut the door. (He removes Huw's cap with a flick of his stick.) Your boots are muddy.

Huw is ordered to address the master as "sir," threatened with a throttling by the teacher's stick, and sent to sit on the dunce's stool. During midday recess in the schoolyard, Huw is further insulted and taunted by a bullying Mervyn Phillips (Clifford Severn) who repeats the master's insults: "Come here, you dirty little sweep." His pencil box is broken - Huw swings at the older boy to defend himself, but he is no match - he is beaten up and knocked unconscious by repeated blows. Bruised and battered all over his face, Huw returns home later that day and lies to his brothers:

Huw: I fell on the mountain.
Ianto: Did you win, Huw?
Huw: No.

His father, believing that his boy must defend himself, calls for defensive training by the town's prize fighter Dai Bando (Rhys Williams) and his partner Cyfartha (Barry Fitzgerald). Huw is willing to return to school the following day, and in recompense, is given "a penny for every mark on your face, a sixpence for a bloody nose, a shilling for a black eye, and two shillings for a broken nose." Huw's mother objects to her husband's encouragement of violence: "Another beating like that and he'll walk home dead." Ianto has fetched Dai Bando and Cyfartha from the Three Bells public house: "Dai Bando is going to teach you to box, Huw." Beth Morgan fiercely objects to the whole school experience: "Baths full of holes, and now prize fighters."

Huw returns to school and during another recess period, he assumes the correct boxing stance of a fighter as he squares off against Mervyn. Although Huw repeatedly knocks down his opponent, the savage teacher grabs him and condemns his rule-breaking fighting: "So our little coal mining friend has been indulging in his favorite sport again, eh?" Huw is whipped across his bare back as the sadistic master canes him. As the painful lashes are rained down upon his flesh, Ceinwen (Ann Todd), one of the schoolgirls, places her handkerchief in the boy's mouth so that he can endure the pain and grip it with his teeth. Huw falls unconscious and the handkerchief drops from his mouth. In the early evening, Huw climbs up his village's hill to return home - his brothers see "the scholar" with his bloodied back from Mr. Jonas' cane: "He has cut you to the bone...," but the young lad doesn't want them to interfere in his own affairs. Ianto proudly carries Huw home: "I think our baby brother is becoming quite a man."

On another day, Huw is startled when Dai Bando and Cyfartha pay a vengeful visit to his classroom teacher, and teach Mr. Jonas (and the schoolchildren) a basic lesson on boxing: "And how would you measure a man who would use a stick on a boy one-third his size? Now, you are good in the use of a stick, but boxing is my subject, according to the rules laid down by the good Marquess of Queensberry...And happy I am to pass on my knowledge to you." With that, Dai jabs his fist at Mr. Jonas' face. He also illustrates how to deliver a "good right hand" - quickly leaving Jonas limp on the floor: "I'm afraid he will never make a boxer. No aptitude for knowledge."

At the mine, staccato blasts of the mine whistle signal an accident, and men and women rush up the village street toward the colliery. Smoke pours from the top of the hill. With a premonition of disaster, Bronwyn looks anxiously up at everyone clamoring toward the summit of the street. The mining cage ascends from the blackness of the mine to the surface carrying a group of miners: "Ivor - fell under a tram, lower level." The musical son Ivor lies dead in the arms of his father. The Welsh miners sing for their lost leader. Mr. Morgan, his sons, and Mr. Gruffydd start down the hill toward Ivor's house - Bronwyn zigs and zags in front of her house and finally her knees give way at the doorstep to her house as she cries out: "Ivor!" That evening in the parlor of Bronwyn's home, a vigil is held. Beth descends the stairs with the news that Bronwyn has delivered a baby in the midst of tragedy:

Mrs. Morgan: We have our first grandson, Gwil.
Mr. Morgan: (solemnly) Give one, and take the other.
Mrs. Morgan: (with a fierce reply) Tell that to that girl up there. She will have an answer for you.
Mr. Morgan: Hisht, now, Beth. Do not kindle the wrath.
Mrs. Morgan: (defiantly) To hell with the wrath. And I'm saying it plain to be heard.

After a number of months of schooling, Huw receives a graduation certificate written in Latin, and is congratulated by his father for making "honors." It is hoped that Huw will somehow attain to something more than coal mining, being a sensitive and intelligent lad:

Mr. Morgan: What will it be? To Cardiff to school? Then the University to be a lawyer, is it, or a doctor?
Bronwyn: (with her newborn baby in her arms) Dr. Huw Morgan. Well, Uncle Huw, that will be something special.

Lonely without her husband, Bronwyn misses Ivor immensely and persistently carries on the daily ritual of putting out her dead husband's clothes: "I put his boots and clothes ready every night, but they are there, still, in the morning. There is lonely I am." As a traditionalist, Huw wishes to remain in his village and work in the colliery with his father, to affirm his father's lifestyle: "I will go down the colliery with you, sir." [With infant in her arms, Bronwyn stands in the background in the doorway with his mother.] His father, knowing that the colliery is no place for Huw, argues that his son should "try for a respectable job" and find a different way of life - "I only want the best for the boy":

I am thinking of the boy's future. It was different in our day. There was good money and fair play for all. But Huw is a scholar. Why take brains down a coal mine?

But Huw remains steadfast in his decision. His father puts on his bowler hat and announces in reaction where he is going: "To get drunk." Huw visits with his sister-in-law Bronwyn and in a noble gesture to take the place of his dead brother, he suggests living in the house with her ("I will get my bed") and having her live off his wages. Huw joins the mine workers as a breaker boy, pushing the trams with coal back and forth on tracks deep underground. At the end of the shift, he takes in a deep breath as he exits from the cage into the sunlight and he happily picks up his meager wages which are distributed from the booth. But his triumph is upstaged when his brothers Davy and Ianto are discharged from the mine - they each receive slips of paper with their money:

And so, it came to Ianto and Davy, the best workers in the colliery, but too highly paid to compete with poorer, more desperate men.

The two laid-off workers pack and bid their mother goodbye, and as their father reads Psalm 23 from the Bible (ironically reciting "my cup runneth over"), they depart with bundles of their belongings.

In a page of an atlas of the world spread out on the Morgan parlor table, Huw draws lines from Wales to America, over to Cape Town, then back to Canada and down to New Zealand - all the locales of the far-flung, scattered Morgan family - but his mother denies the fact that her sons have dispersed:

Huw: One line to Owen and Gwil, down to Cape Town to Angharad, over here to Canada to Ianto, and down here to Davy in New Zealand. (To his mother) And you are the star, shining on them from this house, all the way across the continents and the oceans.
Mrs. Morgan: All the way? How far am I shining, then, if you can put it all on a little piece of paper?
Mr. Morgan: Oh no, a map it is, my old beauty. A picture of the world to show you where they are.
Mrs. Morgan: (grimly) I know where they are - without any old knots or scratches or spiders or pencils. They are in the house.

"Then Angharad came back from Cape Town without her husband. She did not come to us, but stayed at the big Evans house - her house on top of the hill." Huw walks tentatively up the path to the front door of the gigantic mansion, reaches up and uses the great door knocker high above. He is let in by Enid, the maid - in the hallway, the black-dressed housekeeper Mrs. Nicholas (Ethel Griffies), brings Huw to the drawing room where his fashionably dressed sister greets him. Both are grown and changed - although Angharad admits: "I look ill and ought to take care of myself." She is inquisitive about the "news" of everyone in town, emotionally catching herself before inquiring about Mr. Gruffydd, although she learns he "is still first up and last to bed":

Angharad: How is he, Huw?
Huw: Not as he was.
Angharad: Is he ill?
Huw: Inside, in his eyes, in his voice. Like you.
Angharad: (rising nastily) Please go home, Huw.

On second thought, however, Huw remains while tea is served - Angharad insists on pouring tea herself in a dispute with the disagreeable housekeeper, who has been with the Evans family for thirty-seven years and detests her independent-minded mistress: "I always did the pouring for Mr. Iestyn's poor mother...A new mistress is like new sheets, yes. A little bit stiff but washings to come." Suddenly in private, Angharad bursts into tears: "Oh Huw, I tried to tell mother but..." In another scene in the kitchen of the mansion with female townsfolk who gossip over their teacups, the back-biting Mrs. Nicholas whispers 'divorce' - the dreaded secret for Angharad's return without her husband - her voice dripping with righteous indignation:

Saying nothing I am, but that is what is in her mind. She is here without her husband, is it? And why? It is because she is in love with this preacher. PREACHER, I said, Mr. Gruffydd it is.

In the colliery, Huw fights with other mine boys for spreading hurtful lies about Angharad and Mr. Gruffydd. Further gossip spreads throughout the town - little groups of people whisper secrets to each other as he walks down the hill toward home from the colliery:

As the slag had spread over my Valley, so now a blackness spread over the minds of its people. For the first time in my memory, our front door was shut tight in the daytime.

A deacon's meeting is scheduled to be held in the Chapel "over Angharad" - to excommunicate her. Although Huw's sister is innocent ("she has done nothing"), Beth grimly acknowledges that the evil gossip which developed in town after she returned has already disgraced her daughter:

Nothing is enough for people who have minds like cesspools. Oh Huw, my little one, I hope when you're grown their tongues will be slower to hurt.

Huw, washed and dressed, enters the crowded Chapel and finds a seat in the rear pew. The deacons, Mrs. Nicholas, and the sanctimonious Mr. Parry are conspicuously present at the front. In a stirring finale, the preacher defends himself, in his last sermon, scornfully speaking out against unfounded accusations against him. He resigns his position and condemns his congregation and the deacons who will meet later to oust him:

This is the last time I will talk in this Chapel. I am leaving the Valley with regret toward those who have helped me here, and who have let me help them. But, for the rest of you, those of you who have only proved that I have wasted my time among you, I have only this to say. There is not one among you who has had the courage to come to me and accuse me of wrongdoing. And yet, by any standard, if there has been a sin, I am the one who should be branded the sinner. Will anyone raise his voice here now to accuse me? No. You're cowards, too, as well as hypocrites. But I don't blame you. The fault is mine as much as yours. The idle tongues, the poverty of mind which you have shown mean that I have failed to reach most of you with the lesson I was given to teach. (He walks to the back of the Chapel to where Huw is seated) Huw, I thought when I was a young man that I would conquer the world with truth. I thought I would lead an army greater than Alexander ever dreamed of, not to conquer nations, but to liberate mankind. With truth. With the golden sound of the Word. But only a few of them heard. Only a few of you understood. (He turns back toward the congregation with scathing words) The rest of you put on black and sat in Chapel. Why do you come here? Why do you dress your hypocrisy in black and parade before your God on Sunday? From love? No. For you have shown that your hearts are too withered to receive the love of your Divine Father. I know why you have come - I have seen it in your faces Sunday after Sunday as I've stood here before you. Fear has brought you here. Horrible, superstitious fear. Fear of divine retribution - a bolt of fire from the skies. The vengeance of the Lord and the justice of God. But you have forgotten the love of Jesus. You disregard His sacrifice. Death, fear, flames, horror and black clothes. Hold your meeting then, but know if you do this in the name of God and in the house of God, you blaspheme against Him and His Word.

The preacher leaves the Chapel, and Huw rises to walk out and follow him. Mr. Parry tries to keep Huw there by confronting him: "Wait - there is a meeting, Master Morgan," but Huw disregards the command.

Mr. Gruffydd begins writing a final note of farewell to Angharad: "My dear Angharad - I am leaving the valley and..." when interrupted as Huw enters to offer his assistance. The young boy is given the preacher's gold pocket watch, a possession of his father's that was given to him as a gift when he entered the ministry. Huw asks if he will visit with Angharad before he departs:

Huw: Won't you see Angharad before you go? She wants you to.
Mr. Gruffydd: No. If I were to see her again, I couldn't find the strength to leave. Goodbye, Huw, and there's a good old man you are.

Immediately, more tragedies are signalled by a warning whistle at the mine, sounding short alarm blasts. People rush from their homes and run up the hill toward the colliery. A fiery blast explodes at the entrance, where the cage comes to the surface. Beth and Bronwyn hug each other at her doorway, with black smoke billowing behind them. Well-dressed, Angharad hurries over the hillside in the smoke toward the site of the catastrophe. A feared cave-in has occurred, and injured men - all coughing and choking with blackened faces - emerge from the cage which has ascended from the depths. Mr. Morgan's grimy, sodden cap is located. Female relatives of the miners are allowed to push forward to help the men off the cage. Beth looks anxiously for word of her husband - she is joined by Angharad and Bronwyn - but things look grim.

Mr. Gruffydd stops short when he turns around and his eyes meet Angharad's. He asks for volunteers to go down into the mine with him and rescue trapped and buried miners deep beneath the ground:

Mr. Gruffydd: Who is for Gwilym Morgan and the others?
Dai (now nearly blind): I, for one. He is the blood of my heart. Come Cyfartha.
Cyfartha: 'Tis a coward I am. But I will hold your coat.

Other miners, including Huw, step onto the cage to join the rescue party. When Gruffydd is the last to get on the platform, Angharad comes up to him, oblivious to everyone around them. She touches his arm and silently whispers: "Come back." He nods that he will. She looks at him with love in her heart and grasps his hand as the cage descends.

In the half-flooded depths of the mine in waist-deep water, Huw wades through and sings out for his father. Pumps are already being used to clear the mine of water. Crawling through a small passageway, Huw finds his father - still alive, but half-covered and pinned in place by fallen rock and timbers. He lowers himself gently into his father's arms, cradling his head in his father's grasp. His father utters his final words: "There's a good old man, you are," and then his arms go limp. Outside the colliery in the daylight, Beth peacefully senses that her husband is dead. She denies his death by telling the other women of the "glory" he has already seen: "He came to me just now. Ivor was with him. He spoke to me and told me of the glory he had seen." Bronwyn looks heavenward with her. But Angharad fears that they won't return. However, the cage does rise as Welsh choir voices grow louder during the scene.

On the second level of the cage, Mr. Gruffydd stands above Huw, who is on the floor of the cage and cradling his father's head in his lap. As the cage ascends into the light, the boy stares blankly straight ahead with desolation in his eyes. Although Huw has lost his innocence, in the final voice-over dialogue of the film, he still looks back fondly and hopefully during the terrible time of tragedy. He retreats into the glow of his purified memories:

Men like my father cannot die. They are with me still - real in memory as they were in flesh, loving and beloved forever. How green was my Valley then.

The film ends with nostalgic, memory-images of happier, more idyllic days accompanied by a crescendo of chorus voices - of the Morgan family at supper time, of Huw's first view of Bronwyn with the double basket on her hip, of Angharad at the gate watching and waving at Mr. Gruffydd and Huw returning through a hillside of blooming flowers, a view of Huw and his father walking hand-in-hand over the crest of a hill, as they did in the film's opening sequence, and a scene of the five brothers in an open field.