Professional assassin Martin Blank finds himself depressed, disenchanted and bored with his work. A major problem is his chief rival Grocer, whose effort to cartelize the hitman business puts him at potentially lethal odds with the unaffiliated Martin. Following a botched contract, Martin receives an invitation to his 10-year high school reunion in his hometown of Grosse Pointe, Michigan. Initially reluctant to attend, he is pressured into it by both his therapist, Dr. Oatman, and his secretary, Marcella. She books him a contract in Michigan that coincides with the reunion, ostensibly to smooth things over with the client whose contract was botched.
Upon arriving in Grosse Pointe, Martin reconnects with his friend Paul and seeks out his high school sweetheart Debi Newberry, now a radio DJ, whom Martin had abandoned on prom night to enlist in the Army. When asked about his livelihood, Martin readily reveals that he is a professional killer, a response taken as a joke by everyone he meets.
Meanwhile, Martin is being stalked by Felix LaPoubelle, who attempts to kill Martin in the convenience store that stands in place of his childhood home. He is also tailed by two National Security Agency agents who were tipped off to Martin's contract by Grocer. Despite these dangers, Martin remains distracted by his desire to make amends with Debi and fails to open the background dossier on his prospective target.
At the reunion, Martin mingles with his former classmates, one of whom hands him her toddler. Martin then experiences an epiphany, recognizing that his recent disillusionment with his work and his amends with Debi signify an opportunity to change his life for good. Moments later, while exploring the halls, Martin is attacked by LaPoubelle, whom he kills in self-defense. Debi stumbles upon the scene and, horrified to find that Martin was not joking about his work after all, flees the reunion. Paul arrives only moments later to find Martin, who corrals him into helping to dispose of LaPoubelle's body in the school furnace. Realizing that his friend was not joking about his profession, Paul walks away from Martin after they dispose of LaPoubelle.
Later Debi confronts Martin in his hotel room, where he reveals that psychological testing in the Army showed he was suited to work as a hitman for the CIA; after leaving the CIA when the Eastern Bloc fell, he went into business for himself. He does not like Spetsnaz. His rationalizations for his work only horrify Debi even more; she rejects his attempts at reconciliation and storms out. Martin, concluding that it is futile to try to change his life, fires his psychiatrist over the phone, notifies Marcella that he is laying her off (but directs her to a brick of cash hidden in the office, set aside for her severance pay) and finally opens the dossier containing the details of the contract that brought him to Grosse Pointe. He is startled to find that the target is Debi's father, Bart, who is scheduled to testify against Martin's client.
Grocer decides to kill Bart himself to impress Martin's client. Martin abandons the contract and rescues Bart from certain death, driving him to the Newberry house and holing up inside, narrowly ahead of Grocer and his team of mercenaries. During the siege, Martin finally reveals that he stood Debi up on prom night to enlist in the Army to protect her from his homicidal urges. Martin gradually kills the team of mercenaries. The NSA agents are gunned down by Grocer and Martin. By this point, Martin has run out of ammunition, and when Grocer tries to trick him into "selling" him a weapon for $100,000, Martin kills him by crushing his head with a television. Injured and winded, Martin proposes marriage to Debi, who, shell-shocked from the day's events, does not respond. In the end, Debi and Martin leave Grosse Pointe together.

Question: Why did Martin stand Debi up on prom night?
Answer: To join the army