A couple makes out in their car in a remote location. The girl suddenly announces she has a funny feeling. Soon thereafter, someone comes to murder the couple.
Reid and Prentiss attend a crime and violence seminar given by the renowned author Patricia Cornwell. While there, Reid meets an attendee just as smart as he is; he's the inventor of a microchip used to detect tumors. This causes Reid to worry that he doesn't use his brains to help people enough.
When the team members analyze the murder case, they suspect the Zodiac Killer might have returned. They travel to San Francisco to the crime scene and discover more similarities between this case and the ones attributed to the Zodiac Killer in the 1960s down to the nine-millimeter gun and the size 10-1/2 military wing boots. Reid concludes a copycat is at play. Morgan and Rossi head to the old investigator's house to cover their bets. They discover crime scene photos just like the ones at the latest scene, which narrows their search down to the numerous fans who visited the officer over the years.
The team speculates where the next crime might take place. Reid works with Garcia to come up with some options while the rest of the team stakes out the park where the murders occurred. There they find a man with a note attached to him addressed to Reid that reads: "You're not as smart as you think you are." The statement gets to Reid, who later admits to Prentiss that he feels like he should have cured schizophrenia by now.
After analyzing a map of the murder scenes, Reid surmises the killer is choosing his locations based on the last four moves of a chess game, which could suggest that one of the men seen playing chess in the park earlier in the episode could be the murderer.
As the team sets up the next stake out, Caleb (who was shown earlier giving an awkward speech at an engagement party) takes his soon-to-be-wed friend, Harvey) out for his bachelor party. He recreates a scene from the Zodiac murders for Harvey so they can pretend to investigate since the two once shared a keen interest in the subject. It turns out that Caleb has chosen Harvey's fiancée as the victim. Reid shows up just in time to talk Caleb out of killing the couple by appealing to Caleb's obvious superiority to his more successful friend, Harvey.
Eventually, Reid feels better about what he's able to accomplish.