Plot summaryFor several years, Tom Winters (Grant) has been estranged from his wife and their three children, David (Petersen), Elizabeth (Gibson), and Robert (Herbert). The film begins as he returns home from Europe shortly after his wife's death. The children want to stay in the country with their mother's wealthy family, including her parents and her sister Carolyn (Hyer), but Tom takes them to Washington DC, where he works in the US State Department.
The children resent their father's presence, and, at an outdoor concert, Robert runs away. He is found by Cinzia Zaccardi (Loren), the daughter of a famous Italian conductor. She too is running away and is enchanted by little "Roberto" and his harmonica. When she brings him back home, Tom offers her a job as a maid, which she eventually accepts.
Carolyn, now divorced from her husband, offers Tom and the children her old guest house, which was supposed to be moved to a new foundation. However, on the way there, they encounter the guest house being towed down the road, and the house is smashed by a train in a comical scene in which the tow-truck driver, Angelo Donatello (Guardino), flirts with Cinzia and accidentally hits Tom's car. Feeling guilty, Angelo sells Tom his leaky, run-down old houseboat.
Once moved in, Tom discovers that Cinzia is unable to cook, do laundry, or make coffee. Carolyn and others incorrectly insinuate that Cinzia's relationship with Tom is sexual while she innocently wins the affection of Tom and the children. Meanwhile, Tom spends his evenings with Carolyn, who is secretly in love with him. On the 4th of July, she tries to embarrass Cinzia by selecting a gaudy dress for Tom to buy for her, but Cinzia transforms it into an elegant evening gown. She looks so beautiful in the gown that Angelo, a confirmed womanizer, cancels a date with her out of a fear of falling in love and proposing to her.
That evening, Carolyn arrives at the boat with Captain Alan Wilson (Hamilton) and his wife. Alan, who is somewhat drunk, jokes about Cinzia's living arrangement with Tom and slaps her on the behind as she serves drinks. She throws a drink on him in retaliation. Tom asks Alan to leave the boat, but Carolyn takes Alan's side, so Tom asks all three of the guests to leave. David cheers Cinzia up, and they make plans to go fishing, but Tom ruins David's plans by inviting Cinzia to his country club. Once there, Tom reconciles with Carolyn and they agree to get married. As he dances with Cinzia, he finally realizes he is in love with her, but she learns of the proposal, becomes upset, and runs away. Tom catches her, and a little while later, David unhappily finds them kissing in a rowboat.
The children don't want Tom to marry Cinzia. David calls her ugly, Robert rejects her as a mother figure, and Elizabeth wants to continue sleeping in Tom's bed with him. Discouraged by this, Cinzia returns home, but Tom follows her, and she gives in after her father (Cianelli) scolds her. The wedding takes place on the houseboat. The children initially refuse to participate in the ceremony, but as it begins, Elizabeth and David join Tom and Cinzia at the altar, and Robert joins them after playing "Here Comes the Bride" on his harmonica.