Four years after the end of Season 3, Julie is now head of the Catharsis squad, investigating unsolved murders by setting up psychological traps to get the perpetrators to confess. Working on several cases, Julie starts to believe that she’s dealing with a serial killer who’s copying the murders of other killers to conceal his identity. To solve this complex case, Julie once again teams up with Maxime. Bob, who is head of the Homicide unit, discovers that he’s working on the same case as Julie. They all have to work together to capture this elusive killer.