Three

Jennifer Peters tracks the Riddle Killer after he abducts her brother Roy. She has just finished writing a book about the nature of serial killers, which the killer takes offense to. She follows his clues and locates Roy, but the car he is trapped in explodes before she can save him.

R.K. calls Kevin and demands he confess a sin or the car will explode. The killer also poses a riddle: 'What falls but never breaks? What breaks but never falls?' Kevin manages to escape the car before it detonates and reports the incident to the police, omitting the supposed sin.

A threat targets Kevin's childhood dog, prompting him to go to Aunt Balinda's house. He cannot save the dog, and the attempted rescue underscores the killer's reach. Afterward, Kevin returns home and reconnects with his childhood friend Sam to start investigating the R.K. case together.

Kevin and Sam decide to tackle the mystery together. Sam solves the first riddle—'night and day'—and they realize R.K. is watching Kevin from his own home, guiding his next moves through clues.

Another bomb threat targets Kevin's fellow student Henry, with a message referencing Romans 6:23. The police remove the device, and Kevin recalls a boy from his childhood whom he locked in a warehouse and left to die, suspecting a connection to the killer.

Papers from Kevin's thesis are scattered around his room as a TV in his refrigerator broadcasts a new R.K. riddle: 'what takes you away but doesn't go anywhere?' He confesses past abuse and fears the boy may return, while Kevin and Sam rush to evacuate a bus on 3rd Avenue before a bomb explodes.

Jennifer visits Aunt Balinda and learns that Balinda has kept her husband and son cut off from the outside world, mirroring how she had shielded Kevin. She also discovers a bloody jacket in a warehouse, hinting at violence connected to the case.

Sam and Kevin receive a recorded message from R.K. while at a hotel, directing them to an empty building and the warehouse number 33369. They go there, and Sam discovers a wall full of enigmatic words that ultimately traps her inside.

Kevin enters the warehouse, finds a bomb, and escapes with Sam. Jennifer and the police arrive to investigate, but Sam leaves and Kevin refuses to cooperate with Jennifer because the killer demanded no police involvement.

Jennifer resigns from the official investigation, saying she no longer counts. Kevin ignores her, continuing to pursue the case on his own terms as the killer's demands continue to haunt them.

A new threat signals Balinda's house will burn at midnight. Kevin rushes to Balinda to try to save her, haunted by memories of his childhood abuse as he fights to prevent another tragedy.

Sam realizes R.K. bears Kevin's handwriting and notices the riddles hinge on opposites, like night and day. She and Jennifer conclude that R.K. could be Kevin's own 'evil half' rather than a separate killer.

In the warehouse confrontation, Slater identifies himself as R.K. and outlines his plan to force Kevin to kill Balinda and take the blame. Jennifer arrives to find Kevin pointing a gun at himself, and it becomes clear that Slater and Sam are figments of Kevin's imagination.

The real R.K. turns out to be the hot dog vendor seen in a crowd photo; he confesses that copycats motivated him to kill and is captured. Kevin is sent to an institution, and Jennifer visits him there in the aftermath.