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'''''Reazione a Catena''''' (also known as ''Bay of Blood'', ''Twitch of the Death Nerve'', ''Ecologia del Delitto''...) is a 1971 horror film by Mario Bava. 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...

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!!This film provides examples of:

* AnyoneCanDie: And does.
* [[spoiler: BigBadEnsemble: 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}}'''
* GoryDiscretionShot: Averted.
* [[spoiler: {{Greed}}: Which kills literally.]]
* ImpaledWithExtremePrejudice: The two boffing kids and [[spoiler: Simone]].
* KillEmAll: And HOW.
* 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]]
* YouthIsWastedOnTheDumb: Some can say this of the four young people who stop [[IncrediblyLamePun for a quick death]].