Les diaboliques (Diabolique)

One of the most suspenseful films ever made, Les diaboliques revolves around a callous schoolmaster, Michel Delasalle (Paul Meurisse), his heiress wife, Christina (Vera Clouzot), and his mistress, Nicole (Simone Signoret). Nicole, a cold-blooded murderess, helps Christina poison and drown her husband. They dump the corpse in the pool of Michel's boarding school, but Christina grows increasingly fearful that Michel is still alive. An investigation of the schoolmaster's death proceeds, but when the pool is drained no body is found. Adding to the mystery is the testimony of schoolchildren who insist they've seen Michel.