When a couple is murdered in Golden Gate Park, the initial investigation suggests that the Zodiac killer is back. As the team investigates the possibility that the most famous uncaught serial killer of all time has returned after forty years, Reid works to decipher a series of clues and codes left by the UnSub. Through investigation and profiling, the team learns that they are dealing with a genius-level psychopath - and unlike in the original Zodiac case, this killer is working fast.