Les Diaboliques (1955, Fr.) (aka Diabolique)

In French director Henri-Georges Clouzot's psychological horror-thriller - one of the earliest films to feature a shocking plot twist in its conclusion; it was adapted for the screen from Pierre Boileau's and Thomas Narceja's 1951 novel Celle Qui N'était Plus (She Who Was No More) [Note: the scary bathtub scene was imitated in The Shining (1980), Fatal Attraction (1987), and What Lies Beneath (2000).]:

• the film's main characters: Michel Delassalle (Paul Meurisse), a despicable and abusive schoolmaster, Christina (Véra Clouzot, director Georges Clouzot's real-life wife) - Michel's mistreated, downtrodden, humiliated, frail and ailing wife/headmistress and the owner of the school, and Nicole Horner (Simone Signoret), a brassy schoolteacher, Michel's mistress
• the deadly love triangle in the curving plotline and the famous shocking twist ending - the two females plotted to drug the miserly Michel and drown him in a bathtub in Nicole's country house (closeby to the school, in an isolated village), and then dump his body in the school's swimming pool; but then Michel's body went missing from the pool, and there were several possible sightings; however, Michel was never killed by them - he had faked his own death - with collaborative help by Nicole - so Christina could be murdered
• in the sequence, when Christina unexpectedly saw Michel's corpse in the bathtub, it caused her (and the audience) to have a fright-induced heart attack, when he rose zombie-like out of a bathtub with half-opened, all-white eyes; she clutched her chest in the vicinity of her heart, fell back against the wall and slid to the floor where she collapsed and slumped over dead

Christina's Heart Attack

• afterwards, he popped the fake eye lenses from his eyes, exited the tub, and checked Christina's arm for a pulse; after unlocking the apartment door, Nicole rushed into his arms for embraces and kisses; their plan was to become rich by selling the "fire-trap" school that he would inherit from his deceased wife
• in the denouement, the two conspirators were immediately arrested by retired private detective Alfred Fichet (Charles Vanel), who predicted that they would be imprisoned for 15-20 years
• in the film's resolution the next day as the school was closing, there was another possible twist regarding the fate of Christina; confused, truth-telling or lying (?) schoolboy Moinet (Yves-Marie Maurin) declared that Christina had just given him back his confiscated slingshot (which he used to break a window) that morning - the film's last line as he walked away: "I did see her. I know I saw her." (translated)
• the film's unique, one of the first of its kind, end-credits 'anti-spoilers' director's statement that advised viewers to keep the film's ending a secret: "Don't be devils. Don't ruin the interest your friends could take in this film. Don't tell them what you saw. Thank you for them." (another translation): "Don't be diabolical yourself. Don't spoil the ending for your friends by telling them what you have just seen. On their behalf - Thank you!"