To meet the objections of his critics that he only writes lightweight musical comedies, George turns to writing dramatic plays at the outbreak of WW I - his attempt is titled: "Popularity - A Drama in 3 Acts" (described in his own voice-over):

One success followed another. But there was one challenge I hadn't met. Critics kept saying that musicals and cheap comedies were all I could write. I could wave a flag, they said, and nothing else. So I wrote a legitimate drama, very deep and very significant. No music, no gags, no flag-waving. I called it 'Popularity.' The title showed how hard I was hoping. I couldn't attend the opening performance because I was appearing down the street in 'The Yankee Prince.'

Worried and anxious that 'Popularity' won't do well, George asks for reactions from his solemn-looking family who just attended the show - they rate his first dramatic theatrical play as a critical failure:

Josie: The audience loved it, every minute of it.
Nellie: The sets were beautiful, George.
George: Why so quiet, Sam?
Sam: The toughest house I've ever seen. Critics walked out at the end of the second act.
George: Come on, Dad, come on, let's have it.
Jerry: Well, there's no use beating around the bush. Pretty bad, George.
Sam: It could have been a lot better.
Jerry: All right, you wrote a bad play. The only thing to do is forget about it. Everybody's entitled to one failure.

Although George wants to fight the reviewers at their own game and put ads in the papers calling 'Popularity' "the greatest show in town," believing that "it will be my word against theirs and the public will believe me," Sam discourages him to not disappoint and fool his supportive public as well as ruin his reputation: "We stuck our necks out and we got clipped." Noticing that they are in front of a Western Union telegraph office, George leads Sam inside where he forcefully dictates a wire to all the NY papers for the next edition to apologize for the flop:

To the theater-going public:
I wrote a play called 'Popularity.' Mr. Harris and I produced that play. In the opinion of people we respect, it is a bad play. In this we heartily concur. (Sam does a double-take) It is a very bad play. I do humbly apologize and ask forgiveness for having presented anything of which you couldn't possibly approve. There will be five more performances. Please miss them. Signed -

Outside, newspaper boys are yelling about how the Lusitania was sunk by a German sub. George purchases a copy of a newspaper with headlines: "SUB SINKS LUSITANIA - Victim of German U-boat Torpedo." The encroaching war brings both of them back to reality: "And we were worried about the success or failure of a show. Now we've really got something to worry about." Another bold and blazing newspaper headline dissolves into view: "WAR IS DECLARED." George remembers how important patriotism is in times of national threat:

It seems it always happens. Whenever we get too high-hat and too sophisticated for flag-waving, some thug nation decides we're a push-over all ready to be blackjacked. And it isn't long before we're looking up, mighty anxiously, to be sure the flag's still waving over us.

An exterior shot of the U.S. Army Recruiting Office introduces the next scene. George stands in line with other volunteers in civilian clothes to enlist - he is recognized by the army recruiting sergeant (a former clerk at the Friar's Club) as the famed "actor - author - composer - and producer." At his advanced age of 39, George is rejected from the service for being too old:

Sergeant: I'm sorry, Mr. Cohan. You're over age - thirty one's the limit now...You've got to be young and tough for this army. It's no picnic. All those hardships, the mental strain, the trenches, the fighting, the marching, the mademoiselles...No, you won't ever be able to stand it.
George: Hardships and physical strain, young man, you don't know what you're talking about. This war's a coffee klatch compared to a season with a musical show. I'd like to see any one of these kids do what I do in the course of an evening's performance and be on their feet at the finish.

To demonstrate his vitality to everyone in the recruiting office, he performs an acrobatic tap dance up and down the room. One of the military men repeats the rejection to George, suggesting that he could better serve his country by raising morale at home. The last two words cue George for his next composition:

Thank you, very much for your entertainment, Mr. Cohan, but I'm afraid we have more need of you here than over there.

George replies: "But that will make the war last a year longer." Outside, George is inspired by the militaristic sounds of a brass band. He whistles a few of the notes again and again - they become the familiar three notes of his new song. On a bare, dark stage, the camera zooms in on George as he composes at a piano - he struggles with the three notes and eventually creates the triumphant, unofficial World War I theme song: Over There.

On an outdoor entertainment stage in an army camp, George entertains the recruits - he plays and sings his new patriotic creation with popular, turn-of-the-century star/song-writer Nora Bayes (singer Frances Langford) before an audience of doughboys:

Over There

Johnnie, get your gun, get your gun, get your gun
Take it on the run, on the run, on the run
Hear them calling you and me, Ev'ry son of liberty.
Hurry right away, No delay, Go today,
Make your daddy glad to have had such a lad,
Tell your sweetheart not to pine, To be proud her boy's in line.
Over there, Over there,
Send the word, Send the word, Over there
That the Yanks are coming, The Yanks are coming,
The drums rum-tumming ev'rywhere.
So prepare, Say a prayer,
Send the word, Send the word, To beware
We'll be over, We're coming over,
And we won't come back till it's over, Over There.

During the show, the lights suddenly go out, but the blackout is circumvented when George petitions for the headlights of cars and trucks to be turned on. For one of the choruses, George - in closeup - shouts directly into the camera - and to the viewing audience - rousing them with "Everybody sing!" to join him in singing the new war tune. The male-dominated crowd produces a surging choral effect and the song quickly catches on and becomes a national hit.

The New York Chronicle headline reads: "CONGRESS CALLS 'OVER THERE' AMERICAN VICTORY HYMN - George M. Cohan Congratulated by President Woodrow Wilson." A montage of documentary footage shows the WWI recruits being shipped to France on transports and scenes of the Western front battleground. Soon, the war is over: "ARMISTICE! WAR ENDS - Hostilities Cease as Germany Signs Complete Surrender." Victory parades during the day, and Broadway lights ablaze at night, signal the end of the conflict:

We'd won the world war. Manhattan went wild with post-war hysteria. But I spiked my shows with pre-war stuff, the sentiment and humor an older America had aged in the wood.

In a continuous circular pan through the crowded streets and lighted marquees of Broadway, the decade of the twenties passes in a trick montage through all the Cohan and Harris productions in theatres - as the lyrics of Cohan's hit music are sung. In 1920: "The Royal Vagabond." In 1921: "Mary." In 1922: "Little Nellie Kelly." In 1923: "The Follies of 1923." In 1927: "The Merry Malones." In 1928: "Billie." Later in the early 30s, "The Tavern," "La Revue," and "Ah, Wilderness."

Still, it was lonely on Main Stem. Mother and Josie were gone. Dad was by himself on the farm and had grown very old. Every night I went to the theater, I expected a phone call. Well, finally it came.

After attending to Jerry on his deathbed in the Cohan's country farm house, the two doctors, Dr. Anderson (Francis Pierlot) and Dr. Llewellyn (Harry Hayden), reminisce about the end of an era:

Dr. Llewellyn: I remember I was a kid in medical college in Baltimore when I first saw the Four Cohans. They were a great act.
Dr. Anderson: Yes they were. I always thought George M.'s sister was the loveliest dancer I had ever seen.
Dr. Llewellyn: I can't help thinking a theatrical era is dying in there. The sister and his mother gone and now the old man. In some ways, I think he was the best performer of the lot.
Dr. Anderson: Well, I'll settle for his age. There never was anything dull about his life either. And he's lived to see his son an American institution. I'd settle for that, too.

George arrives with Mary and Sam for the death-bed scene. Ominously, he is told: "I think you'd better go in alone." His father is delirious with drugs, mentioning George's playing of Peck's Bad Boy: "Don't you upstage your mother, I'll whale the tar out of ya." They speak about the final curtain call - George weeps as he delivers the 'curtain call' on his father's life and collapses into his father's arms. [His mother and sister died in earlier off-screen scenes.]:

Jerry (feebly): How many curtain calls did you take tonight?
George: Six - six curtains.
Jerry: That's pretty good for a drama. Did you make a speech?
George: (with an unsteady, breaking voice) I thanked them for us. I said 'My mother thanks you, my father thanks you, my sister thanks you, and I thank you.' (His father cups his hand on George's head to comfort him and then expires.)

Bold Variety headlines publish: "BOMBSHELL HITS BROADWAY! - Cohan & Harris Part Company - Famous Partnership Dissolved After 15 Years of Sensationally Successful Play Producing." The theatrical partners dissolve their partnership, but remain friends:

You know, when Dad died, with him went the last link with the Four Cohans. All the back-slapping and the applause, well, it became unimportant. No more fun in it. Didn't mean anything anymore with Dad, Mom, and Josie gone. And so, Mary and I are going away and enjoy ourselves. Visit all those places we've always wanted to see.

Mary and George sail to Europe on their way to a trip around the world, illustrated in a montage of vignettes within steamer trunk stickers:

Life was less full, but it was by no means empty. I still had Mary, a playmate as well as a helpmate. We set out to rubberneck at the world. [In London on a carriage: "Nelson Monument. It's a good thing I wasn't born an Englishman. With the history their flag has, I would've waved myself to death." In Switzerland on a sleigh: "(Yodeling) I learned it on the farm. Nothing but hog calling with frost on it." In Hong Kong on a rick-shaw: "It takes two men to impersonate a horse on Broadway. And you always have trouble casting the front end." Back in New York: "But folks always come back to where their heart is. We came back to the farm, the farm we Cohans had dreamed of when farmers were envying us."]

Now retired, George reclines on a hammock at the farm, reading a Variety newspaper with headlines: "STIX NIX HIX PIX!" which George terms "show-business talk" and then translates: "Small towns...refuse...rube...pictures." Two naive teenagers who drop by are too young to remember him (they were "raised in a vacuum bottle") or know that he was "on Broadway in the legitimate theater." Later that evening, Mary tries to soften the blows to George's irritated pride: "You've been away from that theater for years, George, years. Nine or ten generations have grown up since then. Naturally, they don't remember you."

"To smell that greasepaint again," Mary suggests that for his own good, George should come out of retirement and appear as the lead in a new show that Sam is producing by Kaufman and Hart: "Sam said it's a great part and no other actor in the world but you could do it." To help his old friend and to trick his loving wife, George had already agreed to play the part of then-President Franklin D. Roosevelt in I'd Rather Be Right without her knowing. In the show, the elderly star performs another of his inimitable song and dance numbers: Strictly Off the Record.

The film returns to the present for the final scene in FDR's office, where George is presented with the Congressional Gold Medal (not the Medal of Honor) for his patriotic services in writing the songs Over There and You're a Grand Old Flag. [Congress' official description of the Congressional Gold Medal, authorized to be presented by the President by an act of Congress dated June 29, 1936, was awarded "in recognition of his services during the World War in composing the patriotic song Over There, and prior thereto that thrilling song A Grand Old Flag."] George invokes his oft-repeated curtain call 'thank you' as gratitude for the honor:

George: And then came your wire. I was really worried. Well, here I am goin' on like Tennyson's Brook giving you the story of my life. I'm sorry. I didn't mean to do that. You should have stopped me.
President: Why, I wanted to hear the story of your life. It has a direct bearing on my sending for you. Do you know what this is?
George: The Congressional Medal of Honor.
President: Let's see what the inscription says: 'To George M. Cohan, for his contribution to the American spirit. Over There and Grand Old Flag Presented by Act of Congress.' I congratulate you, Mr. Cohan. (He hands the medal to George) I understand you're the first person of your profession to receive this honor. You should be very proud.
George: Oh, I am proud. In fact, I'm flabbergasted. First time in my life, I'm speechless. Are you sure there isn't some mistake?
President: Quite sure.
George: (modestly) But this medal is for people who've given their lives to their country or done something big. I'm just a song and dance man. Everybody knows that.
President: A man may give his life to his country in many different ways, Mr. Cohan. And quite often he isn't the best judge of how much he has given. Your songs were a symbol of the American spirit. Over There was just as powerful a weapon as any cannon, as any battleship we had in the First World War. Today, we're all soldiers, we're all on the front. We need more songs to express America. I know you and your comrades will give them to us.
George: Mr. President, I've just begun to earn this medal. It's quite a thing.
President: Well, it's the best material we could find, what with priorities and all -
George: Goodbye, sir. (They shake hands.) And I want you to know that I'm not the only one that's grateful. My mother thanks you, my father thanks you, my sister thanks you, and I assure you, I thank you. And, uh, I wouldn't worry about this country if I were you. We've got this thing licked. Where else in the world today could a plain guy like me come in and talk things over with the head man?
President: Well, that's about as good a definition of America as any I've ever heard. Good-bye, Mr. Cohan, and good luck.
George: Good-bye sir, and good luck to you.

George's shadow-silhouette grows to enormous proportions as he exits the President's study through the doorway. With a lively step to his walk, he proceeds to the top of the staircase, and pauses at the framed portraits of other great American leaders of the country - Washington and Jefferson. As Yankee Doodle Dandy plays on the soundtrack, the camera (from the foot of the stairs) captures the celebrated song and dance man's jaunty dance down the stairs - midway, he performs a spontaneous, impromptu buck-wings tap dance with amazing agility. [Cagney improvised his surprise 'down-the-stairs' tap dance on the set.] Two black butlers at the foot of the stairs help him with his coat and cane.

Outside, George stops short at the sight of troops of marching men and civilians engaged in a parade down Pennsylvania Avenue in front of the White House - to the tune of Over There. The presentation of the medal for his song coincides with the outbreak of World War II. From the curb, he stands speechless with others who are singing his song. Unconsciously, he steps into the street and marches in step with the soldiers - his cane becomes his rifle on his shoulder. A soldier next to him questions his silence:

Soldier: What's the matter, old-timer? Don't you remember this song?
George: It seems to me I do.
Soldier: Well, I don't hear anything.

George joins in the singing - profoundly overjoyed and exhilarated:

Well, the Yanks are coming, the Yanks are coming....

His influence extends to another era of patriotic flag-wavers as war clouds gather once more. The film fades out on a close-up of his beaming, proud face with tear-stained cheeks.