When seven-year-old "Mike" arrives in New York from Connecticut, she discovers that her grown sister, Barbara "Babs" Ainsworth, has not come to meet her train. Though she presents a brave front, Mike is soon cornered by concerned passersby and a policeman, who escort her to Symphony Hall, where Babs, a bassist, is performing. Mike cannot resist greeting Babs during the concert and annoys conductor José Iturbi when she steps onto the stage in the middle of a piece. Iturbi is ready to fire Babs for the disturbance until his stage manager, "Andy" Andrews, reminds him that with so many of the symphony's male players gone because of the war, he cannot afford to lose anyone. Babs, who was unaware that her aunt was sending Mike, is overjoyed by the reunion, especially as her husband Joe is off fighting in the Pacific and she is alone. After she and her many roommates, all female orchestra members, sneak Mike into their "no children" boardinghouse, Babs faints. Later, Babs's doctor informs a worried Mike that Babs is pregnant and instructs her to take good care of her sister. Although Mike and her roommates have been sworn to secrecy about Babs's condition, their obvious concern for her quickly alerts Iturbi, who is surprisingly sympathetic. Just before the orchestra is scheduled to leave for Florida on a tour, a telegram arrives for Babs at the boardinghouse. Babs and Mike are out, so harp player Rosalind, one of Babs's roommates, reads the message herself. As feared, the telegram contains bad news about Joe, and Rosalind and the others decide not to tell the fragile Babs anything about it until after the baby is born. On the train to Florida, however, Mike finds Babs quietly sobbing in her berth and learns that she has not heard from Joe in four months and is convinced that he is dead. Mike, who loves to pray to St. Christopher, urges her doubting sister to have faith in God and pray for Joe's return. Soon after arriving in Florida, Babs catches Rosalind yelling at Mike for accidentally spilling the contents of her purse and then notices Rosalind nervously pocketing the fallen telegram. Later, Babs accuses her friends of hiding something from her, but Rosalind insists that the telegram was for her. Despite assurances from Mike, Rosalind and the others, Babs remains doubtful about the future and confesses her fears to Iturbi, who tries to cheer her up with his music. Upon returning to New York, clarinet player Marie, who along with the other women, fears that Babs is going to miscarry, asks her uncle Ferdinand, an alcoholic, petty crook who goes by the name "Bish," to forge a letter from Joe. Bish agrees to compose a letter in which "Joe" reveals that he has been lost on an island for four months, and the next day, an official-looking letter arrives for Babs at the boardinghouse. Babs is relieved to read that Joe is alive and well and rushes to church with Mike to give thanks. Babs's joy makes Rosalind and the others feel especially guilty, but they remain determined to keep their secret until the baby arrives. Later, before a concert, Babs goes into labor, and Mike and her reluctant new friend, Andy, wait at the hospital for the baby's arrival. As they are about to go on stage, Marie and the others, meanwhile, find out from a chagrined Bish that the forged letter was never written. Overjoyed to learn that Joe truly is alive, the women perform Handel's Messiah with gusto and join Iturbi in rejoicing when word comes from Mike that Babs has given birth to a boy.