The Winslow Boy is a play detailing the accusation of a theft by a boy, his expulsion from naval college, and how his family fights to clear his name. According to the script, the action takes place over a two-year period before World War I.

Ronnie Winslow is fourteen years old and a cadet at the Royal Naval College at Osborne. He arrives unexpectedly at home and is greeted by Violet, a maid who is surprised and pleased to see him. Ronnie is nervous and asks where his parents are; they're at church. His parents, Arthur and Grace, and his siblings, Dickie and Catherine, come back from church and discuss recent happenings. Then Catherine's beau John arrives and asks Arthur for her hand in marriage.

Ronnie explains to his siblings that he's been expelled for stealing. Desmond, the family's lawyer, arrives and finds out that Catherine and John have become engaged. The family reads the letter that says Ronnie was expelled for supposedly stealing and cashing a five-shilling postal order. He maintains his innocence and his family believes him.

Nine months later, there's been no progress in clearing Ronnie's name. It took them months to even be able to view the evidence and then they couldn't convince the Admiralty to give them a court-martial and overturn what they considered an unfair ruling. When they finally succeeded in getting the second inquiry, Ronnie was found guilty again. The family is losing money; Catherine won't be able to pay a dowry to her fiancé and Dickie has to withdraw from Oxford and start working at a bank.

Catherine and Arthur argue over whether Sir Robert Morton is the best possible lawyer to take up Ronnie's case. She doesn't like him because he spoke against women's equal rights, which Catherine is passionate about. When he comes to visit and discuss the case, he and Catherine discuss another case he worked on where the person was, according to Morton, guilty.

Robert Morton questions Ronnie and finds out that he wasn't positively identified by the woman who cashed the postal order out and that he insists he only cashed his own postal order. Ronnie also admits that he and Charles Elliot, the boy whose postal order was stolen, were good friends and that he practiced doing his signature for a joke. Morton questions Ronnie hard to make sure that when he's questioned at another hearing, he'll be able to maintain his story.

Arthur and Grace argue over the case. She says that Ronnie is happy at Eton and no one would have had to know about what happened with the postal order except that Arthur continued making it such a large problem and now it's common knowledge.

Time passes and Morton fights on Ronnie's behalf. Catherine even says that no one could have fought a harder fight for them than he has. When Arthur starts to waver on whether they should continue, Morton insists they press on. Catherine's fiancé's father insists that they end their engagement unless Arthur ends the case. John and Catherine talk; she says she wants to marry him and implies that they'll drop the case. When they find out that the case can now go to court, though, she says to let it happen and John leaves in anger.

Five months later, the family reflects on the case. They say that Ronnie was interrogated in the witness box for two days. Morton was key in the trial; he was ruthless with the witnesses. John and Catherine broke up. Arthur has physically deteriorated and may be going into a nursing home soon....