Simone Signoret plays Nicole, a boarding school teacher carrying on an affair with the abusive principal, Michel. After their latest fight leaves her with a black eye, Nicole recruits Michel’s long-suffering wife, Christina, in a plot to kill him. Major spoilers follow.

The plot plays out Hitchcock style. The two women travel to Nicoles’s home town, knowing Michel will follow. When he does, they drug and kill him. Then they must sneak the body back to the boarding school where it they stage it as an accident by dumping it in the pool.

Operating with an icy detachment, Nicole proves the mastermind. Christina abets with a growing reluctance.

Things get complicated when the body vanishes and Christina crumbles under the guilt. Soon a retired detective—best described as a French Columbo—inserts himself into their lives.

It builds to a gothic horror finale that sees Christina creeping along shadowy corridors in pursuit of a phantom-like presence, culminating it a memorable bathtub scene.

But by this point, I had lost sympathy for Christina. Her character exhausted my patience. Lacking any agency, her performance comprises lots of whines and sighs. When she takes initiative in the finale and pursues the shadowy figure, it feels uncharacteristic and forced.

But Diabolique packs a surprise ending that—along with Signoret’s Hitchcockesque ice-blonde performance—distracts from these shortcomings. After watching, I noticed the synopsis on HBO Max gives it away.

Still, I suspect repeat viewings will suffer. Without the twist to distract, the plot holes gape. For me, the biggest surprise wasn’t the twist, but the realization that The Tingler lifted its most memorable sequence from this film.