On New Year’s Eve 1946, Sheila Page, Joan Leslie, stands over her dead husband Barney Page, Louis Hayward, with a gun in her hand. She panics and arrives at a party, seeking help from two friends. There, William Williams, Richard Basehart, suggests she should see the theatrical producer John Friday, Tom Conway. She goes to Friday’s apartment, and as she ascends the stairs, she wishes she could live 1946 all over again. William suddenly disappears, and she is transported back to New Year’s Day 1946. At Friday’s apartment, she decides not to perform in the London production of Saying Goodbye, which baffles Friday.
She returns to her own apartment and hosts a New Year’s Day breakfast with friends, where Sheila warns William to avoid Eloise Shaw, who will be committed to an asylum. The party is crashed by English playwright Paula Costello, Virginia Field, who flirts with Barney. This sparks an argument with Sheila, and Barney goes out to drink. The next morning, Barney comes home drunk, and the pair’s tensions intensify.
Determined to keep Barney from seeing Paula, Sheila travels with him to California. There, Barney reads Paula’s new play Saying Goodbye, which Sheila remembers performing in the original timeline. She initially refuses to perform, which frustrates Barney, who accuses her of wasting time on others’ works instead of finishing his own. Friday arrives and urges her to take the stage, and she consents to perform in the production. During rehearsals, Barney arrives with Paula, and Sheila tries to push Paula to rewrite the final act.
As the play grows into a yearly ritual in New York, Barney and Paula’s affair continues to unfold offstage. At a Thanksgiving party, Barney drinks heavily, insults Sheila, and kisses Paula on the balcony. When discovered, Barney loses his balance and ends up immobilized by a brain paralysis, prompting Sheila to quit the show to care for him. Eloise Shaw later informs William that he has been committed to an asylum. There, William recalls Sheila’s earlier statements about reliving 1946 and is reminded that he walked out of the asylum on New Year’s Eve.
On Christmas Eve, Sheila returns to reprise her stage role, leaving Barney alone with their housemaid. Paula arrives at their apartment, and Barney contemplates leaving with her for London. Sheila returns and confronts Paula as she departs. On New Year’s Eve, Sheila finishes her performance and asks Friday to accompany her to the apartment. There, she receives a note from Barney stating he has left for Paula and can walk again with a cane. On the ship, Barney finds Paula’s stateroom, but she rejects him.
Angered, Barney returns to their apartment and confronts Sheila, intent on killing her. As Barney raises his fist, he is shot dead by William. Police officers arrive and arrest William. As he is led away, William realizes that while certain actions can be altered, the end results remain the same.
