Jennifer Peters, Kelly Brook, is a writer who has just published a book about serial killers. She becomes entangled in a deadly game when her brother Roy is abducted by a sadistic killer known as the Riddle Killer, or R.K., who bases his murders on riddles. Following the clues left by R.K., Jennifer tracks the trail and discovers Roy, but can’t save him in time—the car he is trapped in explodes.
Kevin Parson, a seminary student, receives a chilling call from R.K. demanding that he confess a certain sin or his car will blow up. The killer also poses a riddle: "What falls but never breaks? What breaks but never falls?" Kevin barely escapes the car explosion and alerts the police, including Jennifer, but he omits the part about the sin, chalking it up to something he did years ago.
A separate chain of threats unfolds when Kevin’s childhood dog is targeted by a bomb. He travels to his Aunt Balinda’s house but cannot save the dog. Back home, he reconnects with his longtime friend Samantha Sheer, and the two set out to unravel the R.K. mystery, noting that the killer sometimes seems to be able to monitor Kevin from within his own home. Sam solves the first riddle; the answer is the pairing of night and day.
A new threat emerges when a bomb is attached to Kevin’s fellow student Henry, and a message written on Henry’s forehead cites Romans 6:23—the wages of sin is death. The police manage to remove the bomb. Kevin recalls a boy from his past whom he left behind in a warehouse, believing he died. He thinks the boy survived and might now be R.K.
Kevin finds his thesis papers scattered and discovers a TV in his refrigerator that shows R.K. delivering another riddle: "what takes you away but doesn't go anywhere?" He confesses his past lapse, hoping to placate the killer, but R.K. persists. Kevin and Sam manage to evacuate a bus on 3rd Avenue just before an explosion.
Jennifer visits Aunt Balinda and uncovers a bloody jacket in the warehouse, while Sam talks with Kevin in a hotel. R.K. sends a recorded message pointing to an empty building and the warehouse, and the two race to the site. In the warehouse, Sam sees a wall of enigmatic words and becomes trapped. Kevin enters, finds a bomb, and escapes with Sam while Jennifer and the police arrive to investigate. Sam leaves, and Kevin insists on not cooperating with Jennifer, as the killer demanded no police involvement or more deaths would follow. Jennifer resigns from the case, but Kevin ignores her stance.
Kevin then faces a new threat: a house fire at midnight where Aunt Balinda is in danger. He is reminded of the abuse she inflicted on him as a child. Sam notices that the handwriting on a note from R.K. matches Kevin’s own, leading them to suspect that Kevin's “evil half” may be involved. They realize the riddles point to opposites, like night and day, and conclude that R.K. could be a part of Kevin himself.
A dramatic turn occurs when Balinda is tied up and the killer is revealed as Slater. Sam looks through the door and confirms that Slater is real when she sees two pairs of shoes. Slater explains his plan: he intends to force Kevin to kill Balinda and then pin the crime on him, prompting Jennifer to confront Kevin with the fact that no police involvement will save anyone. Jennifer arrives to find Kevin pointing a gun at himself. It turns out that both Slater and Sam are figments of Kevin’s imagination—hardened by Aunt Balinda’s abuse, he had imagined his friend Sam and the boy from his past, and he had unconsciously mimicked the real Riddle Killer. Jennifer helps Kevin see the truth, and his visions of Slater and Sam vanish.
In a final twist, Jennifer discovers a photo of a hot dog vendor at the front of the crowd, holding a camera and clutching a book allegedly given by the killer to her. The vendor confesses that he hates copycats and had intended to kill Kevin for copying him.
The aftermath leaves Kevin placed in an institution, with Jennifer visiting him there, as the case closes and the line between memory and malice remains deeply blurred.
