The Playboy of the Western World takes place in a run-down pub in the countryside of the North West of Ireland in the early 1900s. The pub’s young barmaid, Margaret Flaherty, better known as Pegeen Mike, is making a list of items she needs for her upcoming wedding to Shawn Keogh, her second cousin. When Shawn comes into the pub, Pegeen tells him of her worry of being left alone in the pub all night—her father, the pub owner Michael Flaherty, is going to be at a wake. Shawn is too afraid of what the local priest, Father Reilly, would think if he were to stay overnight with Pegeen, especially as he needs permission from the priest to approve their wedding between cousins. Pegeen berates him for being so god-fearing, and Shawn makes things worse when he lets slip that, on his way over, he heard what sounded like a young man “groaning wicked like a maddening dog” in a ditch. Pegeen is exasperated that Shawn was too cowardly to investigate.
Soon, Michael Flaherty comes in with his friends, Jimmy Farrell and Philly O’Cullen. The three men are about to go to the wake, an occasion that will last all night and involve a lot of drinking. Pegeen tells her father of her fear of being alone during the night. Michael, Jimmy and Philly try to convince Shawn to stay over, who dodges past his future father-in-law and runs out of the pub.
Shawn soon returns, scared that the man from the ditch is chasing him. That man, Christy Mahon, comes in. He is tired, frightened and dirty, and on the run from the law. Michael, Jimmy, Philly and Pegeen interrogate Christy about the nature of his crime, which he eventually reveals to be patricide—murdering his father. He explains that he killed his father by striking him over the head with a loy when they were in a potato field. Assuming that he must have had good reason to kill his “da,” the locals are mightily impressed by Christy’s courageous deed. Sensing an opportunity, Michael offers Christy the vacant job of “pot-boy,” which will mean Pegeen has someone to keep her safe overnight. Michael, Jimmy and Philly go to the wake. Shawn, now worried about Christy’s presence in the pub, offers to stay—Pegeen tells him to “go on then to Father Reilly.”
Left alone, Christy tells Pegeen more details about his life and the murder of his father, describing a life of rural drudgery and his father’s tyrannical character. During this conversation, she calls him handsome, and the two develop an attraction towards each other. Soon enough, Widow Quin, a thirty-year-old woman who killed her husband, arrives at the pub. She has instructions from Father Reilly and Shawn to take Christy with her back to her house, an idea that Pegeen fiercely resists. The two women squabble over Christy until he eventually insists that he will stay at the pub. Widow Quin leaves, and Christy, in his first comfortable bed for a long time, feels “great luck” at his new situation, wishing he had killed his father sooner.
Act Two takes place the next morning. Christy, still thinking about the attentions of Pegeen and Widow Quin, admires his face in a looking-glass. Four local village girls, Susan Brady, Sara Tansey, Honor Blake and Nelly McLaughlin, come to the pub, excited to catch a glimpse of the young man who killed his father. Christy tries to hide, but they find him and give him gifts from their farms. They notice the looking-glass, which he is trying to hide behind his back, laughing that “them that kills their fathers is a vain lot surely.” Widow Quin comes in and tells the village girls to make Christy breakfast. At Widow Quin’s and the girls’ request, Christy tells the story of how he killed his father, using a chicken bone as a theatrical prop and evidently enjoying the attention. Pegeen comes in and shoos Widow Quin and the girls away. Feeling jealous, she teases Christy by convincing him that the village girls, who she says are often in contact with the “peelers” (local police), might cause the law to come after him. He resigns himself to leaving the pub and moving on, lamenting how he’ll “not be waking near you [Pegeen] another dawn of the year till the two of us do arise to hope or judgment with the saints of God.” She finally gives in and reassures him that he will be safe at the pub. Shawn comes in with Widow Quin and gets Pegeen out of the pub by telling her that her sheep are misbehaving. With Pegeen out of earshot, Shawn offers Christy a one-way ticket to America and his best clothes in exchange for Christy leaving the pub forever, fearing that Christy will get in the way of his marriage to Pegeen. When Christy takes the clothes but refuses the ticket, Widow Quin hatches a plan with Shawn for her to marry Christy in exchange for a reward from Shawn consisting of a ram, a cow, and right of way across his property.
Just as Christy is swaggering around in his smart clothes and enjoying his newfound status, he spots his father, Old Mahon, wounded but not dead, wandering near the pub. Christy frantically hides behind the door as Mahon comes in and asks Widow Quin if she has seen his son, who he describes as a “fool” and the “laughing joke of every woman.” She buys Christy some time by saying she thinks she saw him heading to the coast to catch a boat, sending Mahon off in that direction. Christy begs Widow Quin not to tell Pegeen that his father is still alive. She suggests that he marry her instead of pursuing Pegeen, given that they have murder/attempted murder in common, and promises him a good life. Christy is steadfast in his commitment to Pegeen and asks Widow Quin to help him; she agrees to keep his secret in exchange for provisions from the pub when he marries Pegeen.
Act Three takes place later on the same day. Jimmy and Philly are in the pub discussing Christy’s victories at the village games and sports, and point out how often he mentions his murderous act. Just then, Old Mahon returns. He shows the two men his head wound and explains that it was his son who hit him, arousing Philly’s suspicion. Widow Quin enters, shocked to see Mahon again. She tries to convince Jimmy and Philly that Mahon is a madman who, having earlier said that his wound was inflicted by a “tinker,” changed his story on hearing about Christy Mahon. This persuades Jimmy, but Philly still suspects that Old Mahon might be Christy’s father. Mahon hears cheering outside, which Widow Quin explains is for “a young lad, the champion playboy of the western world.” Mahon takes a look outside, sure that the man in question is Christy; Widow Quin points out that he must be going mad, as he had earlier described his son as a loser—certainly not someone who would be winning the affections of an entire village. Mahon is temporarily convinced that he has gone mad and leaves; Jimmy and Philly go after him.
Christy comes in, surrounded by a crowd of admirers including Pegeen and the village girls. The crowd gives him prizes for winning their sports games. Pegeen gets the others to leave so that Christy can have a short respite from their attentions. Christy, buoyed by his success, convinces Pegeen to marry him, using poetic language to conjure an image of their future together. Michael enters, drunk from the wake and supported by Shawn. After some hesitation, he is convinced by Pegeen and Christy that they should marry, especially by the thought that his grandchildren will become “little gallant swearers” rather than “puny weeds” like Shawn.
Just as Michael joins Pegeen and Christy’s hands together to celebrate their engagement, Mahon comes in for a third time, followed by the crowd and Widow Quin. He runs at Christy and starts beating him. Christy tries to convince everyone that Mahon is a lunatic stranger, but they don’t believe him. They quickly turn on him for having deceived them, with Pegeen especially dismayed at Christy for being “an ugly liar.” Christy, increasingly desperate, chases Mahon out of the pub with a loy. Outside, he deals him another blow, thinking that this one will be fatal. The crowd, led by Michael, are concerned that Christy has now committed murder within their community, and that this will attract unwanted attention from the “peelers” (the police). They decide to hang Christy and bind him in rope. Pegeen, still furious, threatens Christy with fire. Christy fights back aggressively and bites Shawn’s leg.
As Christy is being pulled toward the door, Old Mahon crawls back into the pub. He asks why Christy is tied up, to which Michael apologetically replies that they have to take care of Christy themselves to ensure the safety of the wider community. Mahon loosens Christy’s ropes and insists that his son will be leaving with him. As they leave, Christy states boldly that, from now on, he will be the “gallant captain,” and his father the “heathen slave.” Christy wishes blessings on the pub community, saying that he will “go romancing through a romping lifetime from this hour to the dawning of the judgment day.” With Christy gone, Shawn tries to talk to Pegeen about their engagement, but she just hits him around the head. She pulls a shawl over her and breaks out into “wild lamentation,” crying out after Christy: “I’ve lost him surely. I’ve lost the only playboy of the western world.”