Richard "Dick" Dudgeon is an outcast from his family in colonial New Hampshire. He returns their hatred with scorn. After the death of his father, Dick returns to his childhood home to hear the reading of his father's will, much to his family's dismay. Anthony Anderson, the local minister, treats him with courtesy despite Dick's self-proclaimed apostasy, but Dick's "wickedness" appalls Anderson's wife Judith. To everyone's surprise, it is revealed that Dick's father secretly changed his will just before he died, leaving the bulk of his estate to Dick. Dick promptly evicts his mother from her home, but also invites his orphaned cousin to stay as long as she wants. At the end of the Act, Dick proclaims himself a rebel against the British and scorns his family as cowards when they flee his home.
While visiting Anderson's home at the Reverend's invitation, Dick is left alone with Judith while Anderson is called out to Mrs. Dudgeon's deathbed. Perceiving Judith's distaste for him, Dick attempts to leave, but Judith insists he stay until Anderson returns. While waiting, British soldiers enter Anderson's home and arrest Dick, mistaking him for Anderson. Dick allows them to take him away without revealing his actual identity. He swears Judith to secrecy lest her husband give the secret away and expose himself to arrest. Anderson returns and finds his wife in a state of great agitation. He demands to know if Dick has harmed her. Breaking her promise to Dick, Judith reveals that soldiers came to arrest Anderson but Dick went in his place. Anderson is stunned. He grabs all his money and a gun and quickly rides away, ignoring Judith's appeals. Judith believes her husband to be a coward, while Dick, whom she despised, is a hero.
Judith visits Dick and asks him if he has acted from love. He gently explains that he does not love her, but has acted out of an unexpected decency. During the military trial, Dick is convicted and sentenced to be hanged. This scene introduces General Burgoyne, a Shavian realist, who contributes a number of sharp remarks about the conduct of the American Revolution. Judith interrupts the proceedings to reveal Dick's true identity – but to no avail: he will be hanged in any case. News reaches Burgoyne that American rebels have taken a nearby town, so he and his troops are in danger, especially since orders from London that would have sent reinforcements were never dispatched. The rebels will send a man to negotiate with the British. The final scene of the play is the public square where Dick will be hanged. Like Sydney Carton in Dickens's A Tale of Two Cities, Dick prepares to meet his death. At the last minute, Burgoyne stops the hanging because the rebel has arrived. It is Anthony Anderson, who has become a man of action in an instant, just as Dick became a man of conscience in an instant. Anderson bargains for Dick's life, and Burgoyne agrees to free him. As the Americans rejoice, the British go away, knowing that they face certain defeat.