The first rule of Twins Club is you don't talk about Twins Club.
The nameless narrator (Joe Mauer) is a traveling starting catcher who suffers from skim milk addiction. His doctor refuses to give him medication and advises him to visit a support group to witness more severe suffering. Mauer attends a support group for Royals fans and, after fooling them into thinking that he is a fellow fan/victim, finds an emotional release that relieves his milk addiction. Instead, he becomes addicted to attending support groups and pretending to be a Royals fan, but the presence of another impostor, Carl Pavano’s mustache (played by Bonham Carter), disturbs him, so he negotiates with the ‘stache to avoid their meeting at the same groups.
After a flight home from a road trip, Mauer finds his apartment "destroyed by an explosion" (in reality, a little dusty, not totally spotless as he prefers it). He calls Drew Butera, a supposed "backup catcher" / soap salesman whom he befriended on the flight, and they meet at a bar. A conversation about grittiness leads to Butera inviting Mauer to stay at his place; outside the bar he requests that Mauer hit him. The two engage in a fistfight, and Joe subsequently moves into Drew’s dilapidated house. They have further fights outside the bar, and these attract a crowd of Twins fans. The fighting moves to one fan’s mom’s basement, where the Twins fans form a "blog".
Carl Pavano’s mustaches accidently shaves itself too short, and telephones Mauer for help; he ignores it, but Butera answers the call and saves it. Butera and the ‘stache become sexually involved, and Butera warns Mauer never to talk to the ‘stache about him. More Twins blogs form across the country, and under Butera's leadership, they become the anti-materialist and anti-sabermetric organization called "The Twins Way". Mauer complains to Butera that he wants to be more involved in the organization, but Drew suddenly disappears. When a member of The Twins Way is injured during a botched head first slide into first base, Mauer tries to shut down the project, and follows evidence of Butera’s national travels to track him down. In one city, a Twins Way member greets Mauer, calling him Drew Butera. Joe calls Carl Pavano’s mustache from his hotel room and discovers that the ‘stache also believes him to be Butera. He suddenly sees Drew Butera in his room, and Drew explains that they are dissociated personalities in the same body. Drew controls Mauer’s body when Joe is asleep, when he needs "days off," and when he attempts to rap, and whenever he displays any hint of personality.
Mauer blacks out after the conversation. When he wakes, he discovers from his friend Justin Morneau that Butera made calls during his blackout. He uncovers Butera’s plans to win games through arson and poor lineup construction. Joe Mauer tries to contact the front office, but finds that they are all members of the Twins Way. He attempts to change the starting lineup, but Butera subdues him and moves to a safe building to watch the destruction. Mauer, held by Butera at batpoint, realizes that in sharing the same body with Butera, he himself is actually holding the bat. He hits a portrait of Nick Punto with the bat, shattering it. Butera collapses in shock at this sacrilege against the patron saint of grit, and Mauer stops mentally projecting him. Afterward, the Twins Way members bring a kidnapped Carl Pavano’s mustache to him, believing Mauer to be Butera, and leave them alone. The poor lineup detonates, losing the game, as Mauer and the ‘stache watch the scene, holding hands.
STEVE HOLM! takes over for Mauer/Butera the next day, and defeats the evil Yankee Empire in a plot remarkably similar to Star Wars.