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 Friday the 13th (1980) and 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:
- Anyone Can Die: And does.
- Asshole Victim: Pretty much everyone who dies except for the heiress, the teenagers, and possibly the entomologist and his wife.
- Big-Bad Ensemble: Simon, Renata, Albert, Frank, and Laura are all the killers in the main plot, while Count Donati kickstarts the plot by killing his wife in the prologue (then promptly being offed himself). This is before even mentioning the children in the epilogue.
- Creepy Child: The killers' kids.
- Don't Go in the Woods: Or swimming in the bay. Or in the cabin nearby. Or... anywhere, really. Just drive away.
- Death by Sex: Two kids. Impaled. But in this one the girl is on top.
- Eldritch Location: Similar to other offerings in the genre that the film helped inspire, the bay and its environs seem to have a malevolent, murderous influence on people, though this falls into the Maybe Magic, Maybe Mundane category.
- Gorn: Comfortably Bava's goriest film which sickened audiences back in the day but is now seen as the forefather of the slasher genre.
- Gory Discretion Shot: Averted. The kills are lovingly photographed in detail.
- Greed: The underlying motivation which kills literally.
- HeelFace Door-Slam: After Simon kills the secretary for the death of his mother, the heiress, it's revealed that he killed all the teenagers for finding Donati's body and made a deal to sell the bay in order to keep it a secret. He seems to show some remorse for his part in all this but is immediately killed by Albert.
- Impaled with Extreme Prejudice: The two boffing kids and Simon.
- Kill Em All: Barring the children, everyone is dead at the end.
- The Killer Becomes the Killed: The Count, Simon, Frank and his lover, and the greedy parents in the finale.
- Machete Mayhem
- Mind Screw: The ending. And most of the movie.
- Peek-a-Boo Corpse: The Count to one of the teen girls. She doesn't last long after it, though.
- Later combined with Mirror Scare when Renata goes to bathroom, puts on the light and sees the pile of corpses behind her from the mirror.
- Video Nasty: Gory and infamous enough to make the list.
- Youth Is Wasted on the Dumb: Some can say this of the four young people who stop for a quick death.