A killer uses enhanced hallucinogens in his methods, causing the team to work with the Narcotics Division and Catherine's former lover. Meanwhile, Thomas must revisit a case from which he based his book, as he receives post cards from the killer. Also, Catherine's mother is hospitalized after apparently having a stroke and falling down the stairs at home. Her condition worsens, and her family must make a difficult decision.

The team investigates the connection the killer has with his drug dealing, as he targets his next couple. Meanwhile, Thomas becomes paranoid trying to think like him. At her mother's funeral reception, Catherine is introduced to a boy being mentored by Howard. Suspicious, she is torn between investigating him and the murder case. This causes her to abandon Molbeck on surveillance and he is shot by the killer.

With Molbeck in the hospital, the police have stepped up their methods to catch the killer. Believing the killer will not change his dumpsites, all city park entrances become checkpoints. His vehicle passes inspection, as he has already disposed of a body. Knowing he can no longer use the vehicle, he allows his current captive victims to escape in it, only after he rigged it to explode. The FBI takes over the case, but Thomas and Catherine set a trap for the killer by publishing he is a copycat killer. They deliver him to Bisgaard. Meanwhile, Thomas tells Catherine to order the 9-1-1 call Howard placed when her mother fell down the stairs. When she listens to the call, she hears Howard speak to a boy in the house. Catherine believes her mother was killed when she discovered Howard raping the boy. Armed, she enters Howard's house, with Thomas pursuing her, while Howard waits inside with a gun. The sound of one shot fired is heard in the closing scene.