It is 1912, and fourteen-year-old Ronnie Winslow has just arrived home after being expelled by Osborne Naval College for allegedly stealing a postal order from a fellow student. He reveals himself to Violet, the Winslows’ housemaid, and seems incredibly frightened. As his parents, Arthur and Grace, and siblings, Dickie, Catherine, arrive back from church, Ronnie quickly runs into the garden to hide in the rain.
As the family enters the house, Arthur chastises Dickie for not working hard enough on his studies at Oxford University (studies that Arthur funds). Dickie complains that he is treated unfairly, and that Ronnie is the favorite son. Grace asks questions of Catherine about her fiancé, John Watherstone, and whether he still wants to marry her even though she is a passionate believer in equality for women. Soon after, John comes to the house to discuss the potential marriage with Arthur and to obtain his permission. After a stiff and formal conversation, Arthur agrees to provide a small dowry.
Now alone with Catherine, John admits how nervous the conversation with Arthur made him. To Catherine’s bemusement, he had planned eloquent phrases in advance, though nothing about how much he loves Catherine; he assumed that Arthur and he could take that as given.
A soaked Ronnie reveals himself to Catherine, who reads the letter detailing his expulsion. Dickie comes in and is instructed by Catherine to fetch Grace and keep Arthur upstairs, so that he doesn’t see Ronnie. When Grace comes down she makes a big fuss over Ronnie; Dickie, meanwhile, can’t believe he’s been expelled over a mere “bit of pinching.” They all agree it’s best to hide Ronnie away from Arthur, fearing his reaction to the expulsion.
Desmond Curry, the family’s hapless solicitor, arrives. He congratulates Catherine and John, though a little awkwardly as he’s been in love with Catherine for a long time. Arthur brings out a bottle of wine to toast the engagement and asks Violet to bring some glasses. He pours everyone a drink, including Violet, who protests that she didn’t want one—her glass was actually meant for Ronnie. Arthur insists that Ronnie isn’t yet due back from college, but it then dawns on him that Ronnie may have been expelled. He makes Grace read him the letter and then orders Ronnie to come downstairs, telling everyone else to leave the two of them in the room alone.
Arthur is hurt to learn that his son would rather hide in the rain than suffer his wrath. He looks Ronnie intently in the eyes and asks him to tell him the truth—did he steal the postal order or not? A tearful Ronnie vehemently denies the charge, and Arthur believes him. Arthur sends Ronnie up to bed and immediately starts making phone calls to try and clear his son’s name.
Nine months later, Dickie and Catherine sit in the living room, reading letters in the newspaper about the “Winslow boy” case. The case has gained a fair amount of media attention and prompts a range of opinions: some think it’s a fundamental fight for human rights, others that it is a massive distraction when the Navy should be focusing on more important things (like Germany’s rearmament). To cheer themselves up, Dickie and Catherine dance to the gramophone; Catherine reveals that her wedding has been postponed because John’s father, a military colonel, is away for six months.
Arthur has a one-on-one conversation with Dickie in which he asks him to assess his own chances of successfully passing his Oxford exams. Because Dickie is less than convincing, Arthur informs his son that, as the legal proceedings are costing a lot of money, he will no longer be able to fund Dickie’s studies and he will have to leave Oxford.
Arthur reveals he is engaging the services of Sir Robert Morton, the most respected barrister in the country. Arthur asks Catherine if they’re “mad” to pursue the case and reveals that he will not be able to provide the planned dowry, but Catherine has already come to terms with that. She reveals that she doesn’t have much faith in Sir Robert as she fundamentally disagrees with his politics and thinks he is self-serving.
Sir Robert arrives with Desmond. He informs Arthur that he will be seeking a “Petition of Right”—a complex legal mechanism that gives an individual express permission to sue the Crown (essentially the state). He summons Ronnie down and subjects him to an extreme interrogation. This greatly distresses Ronnie, who admits having practiced the signature of his fellow student and can’t remember all the details of what happened on the day. Sir Robert accuses him of being guilty and causing pain to his family, which Ronnie vehemently denies. Just as Catherine and Arthur start to object to Sir Robert’s technique, he calmly goes to leave, agreeing to take on the case as “the boy is plainly innocent.”
Nine months later, the media attention on the case has intensified and it’s now being discussed in the British parliament. Arthur sits in his armchair, reading an account of the debate to Ronnie and Grace, who are trying to stay awake and darning socks respectively. Grace thinks Ronnie should just go to bed, but Arthur thinks he ought to listen to the news about his case. Arthur also tells Grace that Violet needs to be dismissed as they can no longer afford her. Grace doesn’t want to do that and tearfully accuses Arthur of sacrificing their lives for the sake of the case.
Catherine comes back from watching parliament and fills Arthur in on the day’s proceedings. Catherine is grudgingly impressed by Sir Robert, who to her shock walks in through the door at that very moment. Arthur reads a letter brought in by Violet while Catherine and Sir Robert discuss the latter’s earlier interrogation of Ronnie. Having read the new letter, Arthur suddenly insists that they must drop the case. Catherine snatches the letter—it’s from John’s father, saying that, unless the Winslows drop the legal action he won’t be able to endorse the marriage; as a colonel, he finds their attack on the military establishment embarrassing and unpatriotic. John arrives and almost convinces Catherine that they should drop the case. But at that moment, Sir Robert receives a phone call saying the “Petition of Right” has been granted, meaning the case can actually go to court and have a public trial. Arthur says it’s up to Catherine if the case proceeds. Without hesitating, she says that it must. John storms out.
Five months later, the case is now one of the most talked about issues in the country; the Winslow telephone is always ringing, and the house is surrounded by journalists. Dickie comes in, having just returned from Reading where he now works in a bank. Grace calls Arthur, who is sitting in the garden, to come in for his lunch, before telling Dickie how exciting she finds the court case—she’s never seen “such crowds,” though she doesn’t really follow the proceedings very well. Dickie asks how Catherine is doing as he’s heard John broke off the engagement. Arthur comes up the stairs and gets into a wheelchair; his health is getting worse.
Arthur goes into the dining room for his lunch, and Grace reveals to Dickie that he is due to go into a nursing home after the trial, though she doubts he actually will. Catherine arrives from court, relaying the information that Sir Robert is now worried about the outcome of the case. Grace and Dickie go to court, leaving Arthur and Catherine to have a heart-to-heart. Catherine reveals increasing admiration for Sir Robert, but both are concerned they will lose.
Desmond Curry comes to the house. Having heard of Catherine’s split from John, he seems like he is about to propose to her, even though he admits she will never love him. She cuts him short and just says she will think it over, and he leaves confused—but not before letting slip that Sir Robert turned down a prestigious job as a judge to continue working on the case. Arthur comes in, complaining of being gawped at through the front window. Catherine confesses she is considering marrying Desmond so she can have a comfortable life and might give up working for women’s suffrage. Arthur thinks marrying Desmond would be lunacy. Arthur kisses her on the head and apologizes for messing up her life, but she says they’ve only done what they must do in the face of “tyranny” and “injustice.”
Suddenly they hear a shout from a newspaper boy outside: “Winslow case result!” Violet comes in and tells the Arthur and Catherine that they have won , describing jubilant scenes in the courtroom. Arthur and Catherine are taken aback and a little annoyed to have missed the victory.
Sir Robert comes to the house and gives Arthur a scrap of paper that has written on it the Admiralty’s statement absolving Ronnie of all guilt. He tells Catherine it was a pity that she wasn’t in court. Arthur goes outside to make a statement to the press, insisting on standing up rather than being wheeled out. Catherine quizzes Sir Robot on why he is so emotionally guarded. He admits he cried at the verdict, as “Right” had been done. Ronnie comes in and apologizes for missing the verdict, asking if they won. Sir Robot says he hopes to see Catherine in parliament again; she says that if she does it will be as equals, not her watching him work. As he leaves and bids her goodbye, he doubtfully replies, “Perhaps.”