In a Halloween themed episode, Five-0 investigates the murder of a nursing student named Lisa Kerner, who was kidnapped and cut to pieces while she was still alive. Initial evidence seems to point toward selling Lisa's organs on the black market as the killer's motive, but Max performs an autopsy and finds that the killer used a weapon engraved with a pentagram, indicating a ritualistic killing. Five-0 traces Lisa's steps prior to the killing and discover that she attended a Halloween party and left with a guy named Lucas, and both were followed by a man in a goat mask. Kono does some research on the occult and theorizes that Lucas could be the second part of the killer's pagan sacrifice. Danny realizes that the killer must've started his sacrifices on smaller animals, and a quick search through the database leads them to Seth Upton. Five-0 storms his grandmother's house, where he lives and has a torture chamber, and grill Seth's grandmother about his whereabouts. The grandmother turns out to be a bit unhinged, herself; she calls Seth to warn him about the cops before taking her own life. Five-0 traces the call and locates Seth at an ancient Hawaiian sacrifice site, killing him before he can perform the second part of his sacrifice with Lucas.