Monk is paying Natalie, who is upset that he isn't paying her for overtime because she didn't complain enough. She obliges by complaining that he still owes her $1,800 in back pay. They're interrupted when a Linda Kloster arrives. She says she's the wife of Patrick Kloster, a grandmaster chess player and qualified genius. She asks for Monk's help explaining that her husband plans to murder someone: herself.
Linda, who admits she's a former alcoholic, explains that Patrick has been threatening her for weeks but that she has no way to prove anything. He has a $10 million insurance policy but is in Vancouver playing in a tournament. Natalie figures she's safe but Linda isn't convinced. She takes Monk's hands and asks him to take the case, and he promises to do it if she dies.
Back home, Linda goes to her bedroom and talks to her housekeeper Paulina, who is concerned for her. Paulina goes downstairs to get her lunch and talks to the cook, but when she returns she finds Linda dead on her bed.
Monk and Natalie are talking to Stottlemeyer to arrange police protection for Linda when Disher arrives to reveal that Linda's murder has been called in. They go to the house and Monk goes over the bedroom but doesn't find anything. The coroner has confirmed that Linda died of natural causes but Monk suspects poison. However, Linda hasn't eaten anything all day and he can't account for her death.
They go to the airplane as Patrick returns from Vancouver. They meet Patrick and ask him to provide a statement. He apologizes for being too busy, but says he anticipated their request and has reserved an office at the airport, and even knew how many people would be there. He admits he was upset when he got word of Linda's death, but simply took longer to win his match. Monk explains that Linda hired her to investigate her upcoming death and suspects poison. Patrick says the only thing he knows about poison is a poison pawn maneuver in chess, then leaves.
Monk, Natalie, and Julie go to the park to consult a chess expert: teenager Eric Tavala, one of the few people to come close to defeating Patrick. Eric charges $20 a game and Monk doesn't have enough money, but Julie offers to play for the discount rate. Eric easily defeats her but is impressed, and explains that a "poison pawn" is the irresistible offering of a piece to lure an opponent into a trap. Monk realizes how Linda was murdered and goes to the funeral home. He finds Patrick standing over a coffin and explains that when he was in Linda's bedroom he saw clues indicating that she was drinking again. Patrick poisoned her hidden stash of alcohol and she poisoned herself, hiding the evidence before she died simply to cover her drinking. Monk plans to have Linda's body thoroughly examined for poisons, but Patrick trumps him revealing that the coffin isn't Linda's. He had Linda cremated several hours ago.
An increasingly obsessed Monk parks outside of Patrick's house with Natalie to watch the genius' every move. They read his chess books, noting an inscription to a beloved Tatiana. Natalie realizes that Monk hasn't cashed Linda's check and he insists that he has failed and refuses to endorse the check. Patrick comes out with drinks and plays the gracious host. Giving up, they prepare to leave but Monk notices that Patrick grows oleander in his garden. He goes to Stottlemeyer and explains that oleander can be used to make a poison that induces heart attack-like symptoms. When Monk suggests that all it would take to incriminate Patrick would be a bottle of poison, Stottlemeyer gets him alone and accuses him of preparing to plant evidence. Monk initially denies it, but when Stottlemeyer pursues the issue and tells him to let it go, Monk explains that Linda took his hand the same way that Trudy did when she was dying. Monk refuses to lose someone else and refuses to move on.
Back in his apartment, Monk grinds up oleander stalks and puts the powder into a bottle, then breaks into Patrick's home. He considers putting the bottle in Patrick's book shelf, changes his mind, turns to go… and Patrick turns on the lights. He noticed the missing plants and knew Monk would try to break in but says he won't bother to press charges. Monk notices a tray with a silverware set: a wedding gift. Remembering the book inscription, Monk realizes that Patrick was married before… and she died too.
Monk and Stottlemeyer go before Judge Barr and explain that Patrick's first wife Tatiana also died of a heart attack, and was also heavily insured. The judge approves an exhumation order for the next day, and they then go to the cemetery and dig up Tatiana's body to check for poison. As they wait for the analysis, a confident Patrick tells them they're making a mistake. The examiner comes out and informs them that there is no trace of poison in Tatiana's body. Patrick, smug, thanks Monk for the game and leaves.
Monk, still obsessed, takes Natalie and Julie to a chess exhibition where Patrick is playing four opponents simultaneously. Patrick refuses to take Monk's calls but Monk wants to confront the genius by playing him. One of Patrick's opponents is Eric, and Julie convinces him to let Monk stand in as his proxy. Patrick makes the first move… and Monk obsesses over the pieces and whether they're centered. Patrick continues to play his other three opponents but is driven to distraction by Monk's obsession. Monk tries to bargain with him, suggesting Patrick turn himself in if Monk wins. Patrick refuses and castles his pieces in one of his other games. Monk notices and Patrick explains that he uses castling as the best way to get out of trouble.
Realizing what happened, Monk goes back to the cemetery and finds signs that a nearby headstone was dug up. He calls n Stottlemeyer and they exhume that coffin. It's Tatiana's: Patrick switched the headstones the night before the exhumation. Monk confronts Patrick and informs him that he's sick of the genius' chess analogies… and then declares "checkmate."
Later, Monk is reading at home when Natalie expresses her admiration for Monk and declares him her hero. She asks for his autograph and he obliges: it's Linda's check. They fight over the check until she points out his books are crooked, then slips out to the bank.