"Chain Reaction"]) (also known as Carnage, Twitch of the Death Nerve and Blood Bath) is a 1971 Italian giallo slasher film directed by Mario Bava.

Bava co-wrote the screenplay with Giuseppe Zaccariello, Filippo Ottoni, and Sergio Canevari, with story credit given to Dardano Sacchetti and Franco Barberi.

Their scheme is dealt a potentially ruinous blow when Filippo's estranged daughter Renata appears, determined to ensure that her father's estate comes into her possession.

Accompanied by her husband Albert and leaving their young son and daughter in a caravan nearby, Renata visits the house of Paolo Fossati, an entomologist living on the grounds of the Donati estate.

Knowing there are now no other living heirs, Albert and Renata prepare to return home to await the announcement of their inheritance when they are shot dead from the caravan by their son, who has mistaken their shotgun for a toy.

The genesis of A Bay of Blood was when producer Dino De Laurentiis heard that Dardano Sacchetti, screenwriter of the popular The Cat o' Nine Tails, had fallen out with the film's director Dario Argento.

In this early version of the story, the parents are driven to commit one murder after another in a chain reaction, becoming so caught up in their plan that they abandon their children for several days.

[1] Sacchetti wrote the first draft of the script, titled Cosi imparano a fare i cattivi ("That Will Teach Them to Be Bad") after a line spoken by the children after killing their parents, with his writing partner Franco Barbieri.

[1] The film began production in early 1971, still under the shooting title Cosi imparano a fare i cattivi,[6] which was soon changed to Reazione a catena ("chain reaction").

[7] Yet another title used during shooting was La baia d'argento ("the bay of silver"), discarded for fear that the movie would be perceived as a parody of Dario Argento's works as a result.

Due to the severe budgetary restrictions, Bava not only acted as his own cinematographer, but also utilized a simple child's wagon for the film's many tracking shots.

To create the deaths of Anna, Brunhilda, and Denise, wax effigies of the actresses' throats and backs were constructed and rigged to expulse brightly colored blood when cut.

[1] The illusion of Bobby being stabbed in the face with a billhook was achieved with a prop blade which was swiftly pulled out of frame to hide the fact that it was sculpted to conform exactly to actor Roberto Bonanni's profile.

[1] The film was announced as Antefatto ("Before the Fact"), but when finally released to theatres, the title had changed to Ecologia del delitto ("Ecology of Crime").

[7] When the film did poorly on its initial release, it was pulled from Italian theaters and retitled Reazione a Catena ("Chain Reaction"),[1] and was later re-released as Bahia de Sangre ("Bay of Blood" in Spanish).

[1] Thanks to Hallmark's distribution partnership with Bava's former employer American International Pictures, the film played for years under this title in drive-ins and grindhouses throughout the country as part of a double or triple bill with other Hallmark/AIP films, most notably Wes Craven's equally-influential The Last House on the Left and Fernando Di Leo's Slaughter Hotel,[1] but it has since been re-released theatrically and on home video under a variety of titles.

If you were appalled by the gore and slaughter in Blood and Black Lace, this latest film contains twice the murders, each one accomplished with an obnoxious detail ... Red herrings are ever-present, and serve as the only interest keeping the plot in motion, but nothing really redeems the dumb storyline".

Assembled with a striking visual assurance that never ceases to amuse, this is typical Bava material, simply one ghastly murder after another – 13 in all – surrounded by what must be one of the most preposterous and confusing plots ever put on film".

It helped kick start the slasher genre … [Bava's] influence still resonates today (although somewhat dully) in movies such as I Know What You Did Last Summer, Scream and Urban Legend".

[21][22] Along with The Burning, Just Before Dawn (Both 1981) and several other similarly plotted slashers, Friday the 13th specifically "followed Bava's inspired cue, having young people stalked by violent death amid beautiful wooded settings".