Ethnologist John Parker loves his two boys, Al and Terry, and misses them terribly when they have to leave his archaeological dig at the end of the summer. While Al goes to school, Terry returns home to their mother Grace, from whom John is divorced. Once home, Terry learns that Grace has married Dr. Phil Shumaker, a stable man who can provide the home that Grace never had with John. When John gets a note from Terry saying that Grace has married and he wishes he were dead, John decides to give up his work and go to his sons. He visits Al at military school first and asks his assistance in helping Terry to accept Phil as his new father. Though the terms of the divorce prevent John from seeing Terry at home, he takes a place near them and asks Al to keep in touch with him about Terry's progress. Though Phil tries to be a good father, he does not understand Terry and makes rules that are impossible for the child to follow. When Al comes home, he tries to convince Terry that Phil deserves a chance, then talks to Phil about being more of a pal to the boy. One night, after the family has gone to the movies, Al and Terry go for a soda and Al slugs an older boy who says something snide about Grace. Al then develops a crush on Lucille Smith, their next door neighbor, and walks her home instead of Terry. The next day, Phil can't go on a promised rowing trip because of a patient, so Al and Terry go alone. At the lake they meet Lucille, whom Terry insists on taking along, much to Al's disgust. While Terry is playing, he suddenly hears John whistle for him and is overjoyed to see his father. John makes Terry promise not to tell anyone about their brief meeting, however. Terry becomes jealous of Al's attentions to Lucille and causes the boat to tip over, after which Al and Lucille chase him away. When Terry returns home without the others, he and Phil have a serious argument that results in Phil giving Terry a whipping. When Al takes Lucille home, her father orders him off their property, because Terry has implied that something improper happened between Lucille and Al. Angered that Terry has caused him to get on the wrong side of Lucille's father, Al refuses to side with Terry in his argument with Phil. Feeling alone, Terry then runs away, taking the dog that Phil had given him as a companion. They go on the river in a rickety old boat, but are shortly thereafter followed by Al, who remorsefully goes to look for him, after an argument with Phil. At John's cabin, Terry gives their whistle signal, attracting John's attention. When Terry gives a slightly distorted version of his whipping story, John decides to go to Phil for a confrontation. As Al looks for Terry on the river, he is hit by an oncoming boat. Meanwhile, as Grace is about to leave Phil over the whipping, they are informed of Al's accident. Because Al's lung is punctured, when Phil arrives he has to operate immediately. When Grace returns for Phil's nurse and supplies, she tells John, who has just arrrived. Back at the cabin, Phil comforts a miserable Terry and promises to save Al. When Al needs a transfusion, both Phil and John volunteer to be donors, but only Phil's blood is in the right group. When Al recovers, John prepares to sneak Terry away with him to South America, but Terry has now come to like and respect a considerably softened Phil and John decides at the last moment to let Terry stay with his mother and stepfather.