'''''Reazione a Catena''''' (also known as ''Bay of Blood'', ''Twitch of the Death Nerve'', ''Ecologia del Delitto''...) is a 1971 horror film by Creator/MarioBava. Designed as a comeback film after he lost his distribution deal with American International Pictures, Bava effectively created a film that has served as the blueprint for the Bodycount/Slasher genre of horror, with many of its sequences copied by other horror films (most notably ''Film/FridayThe13th1980'' and [[Film/FridayThe13thPart2 its first sequel]]).

The film's plot is a convoluted "Who Done It", as people begin dying left and right during the course of a long weekend at the lakeside home of an elderly heiress. There are multiple murderers at work, as a slew of shady characters (and a quartet of sex-obsessed teens) end up dying in horrific fashion. There is also a surprise ending that will shock you...

!!This film provides examples of:

* AnyoneCanDie: And does.
* BigBadEnsemble: [[spoiler: Simon, Renata and Albert are all the killers.]]
* CreepyChild: The [[spoiler: killers']] kids.
* DontGoInTheWoods: Or swimming in the bay. Or in the cabin nearby. Or... anywhere, really. Just drive away.
* DeathBySex: Two kids. Impaled. [[Film/FridayThe13thPart2 But in this one the girl is on top]].
* '''{{Gorn}}''': Comfortably Bava's goriest film which sickened audiences back in the day but is now seen as the forefather of the slasher genre.
* GoryDiscretionShot: Averted. The kills are lovingly photographed in detail.
* {{Greed}}: [[spoiler: The underlying motivation which kills literally.]]
* ImpaledWithExtremePrejudice: The two boffing kids and [[spoiler: Simone]].
* KillEmAll: [[spoiler: Baring the children, everyone is dead at the end.]]
* TheKillerBecomesTheKilled: [[spoiler: The Count]], [[spoiler: Simone]] and [[spoiler: the greedy parents]] in the finale.
* MacheteMayhem
* MindScrew: The ending. And most of the movie.
* PeekABooCorpse: [[spoiler: [[TheKillerBecomesTheKilled The Count]]]] to one of the teen girls. [[spoiler: She doesn't last long after it, though.]]
** Later combined with MirrorScare when Renata goes to bathroom, puts on the light and sees the pile of corpses behind her from the mirror.
* [[VideoNasties Video Nasty]]: Gory and infamous enough to make the list.
* YouthIsWastedOnTheDumb: Some can say this of the four young people who stop [[IncrediblyLamePun for a quick death]].