When Tom Winston, a government lawyer who, for several years, has been separated from his wife, takes charge of his three children--Elizabeth, David and Robert--following their mother's death, he is taken aback by their hostility toward him. His sister-in-law, the attractive but unhappily married Carolyn Gibson, explains that the loss of their mother has left the children with problems: None of them sleeps well, all three are melancholy and little Robert, who claims to hate everyone, does nothing but play the harmonica. After first agreeing to let Carolyn and her parents adopt the children, Tom suddenly decides to take them to nearby Washington, D.C., where he rents a small flat. The children are unimpressed with their new home, and following an evening concert at the Watergate, Robert hides in a rowboat on the adjacent Potomac River. Also attending the concert is the beautiful but restless Cinzia Zaccardi, who is accompanying her father, a famous Italian conductor, on a tour of the United States. Cinzia longs for freedom and male companionship, but her father keeps a tight rein on her, and she escapes a stuffy society dinner only by climbing out a window and into Robert's rowboat. Cinzia dances with the child at a street carnival and later that evening takes him home. Tom threatens to spank the boy until Cinzia gently advises him to be "a parent, not a policeman." Seeing that all three children are taken with Cinzia, Tom, who believes that she is an abandoned "G.I. bride or something," offers her a job as their maid. Amused, Cinzia declines the job and returns to her father, but when he angrily vows never to let her out of his sight again, she decides to accept the job and move with Tom and the children to Carolyn's guest house in nearby Virginia. When the guest house is accidentally demolished, an Italian-American storekeeper named Angelo Donatello offers to sell Tom his rickety houseboat. During the family's stormy first night on the boat, Cinzia sends a frightened Elizabeth to sleep with her father, who slowly begins to treat the child with warmth and affection. Carolyn reveals that she is divorcing her philandering husband and admits that she has always loved Tom. Meanwhile, Angelo invites Cinzia to the Fourth of July dance sponsored by the Sons of Italy. Cinzia and the children work hard to fix up the houseboat, and soon it is homey and charming. David, unhappy about his father's constant criticism, however, decides to run away one windy night. When David's rowboat capsizes, Tom leaps into the river and saves him. Cinzia tries to persuade Tom to be more accepting of David, and as the two talk, they find themselves nearly kissing. The next morning, Tom and David discuss death, and David teaches his father how to fish. Tom begins to date Carolyn, which so upsets Cinzia that she decides to leave. Tom buys her a dress and remarks that she has pulled his family together again. Just then, Carolyn and her friends arrive, and after one of them insults Cinzia, Tom orders them from the houseboat. He then takes Cinzia to the country club dance, and as they kiss at the end of the evening, he realizes he is in love with her. To Cinzia's surprise, the children, especially the jealous David, disapprove of their romance, and after explaining that she could never take their mother's place, she brokenheartedly returns to her father. Tom tracks her down and declares his love in the presence of Maestro Zaccardi, who, although approving of the union, warns Tom never to hurt his beloved daughter. The children, however, do not come to terms with their father's remarriage until the wedding ceremony begins. After playing "The Wedding March" on his harmonica in the middle of the couple's vows, Robert smilingly approaches Cinzia, and the ceremony continues as the children join hands with the couple.