Diabolique

Mia, a devout Catholic schoolteacher, endures an abusive marriage with Guy. Nicole Horner, Mia's fellow teacher and Guy's mistress, becomes protective of Mia. Together they decide to murder Guy to escape his control.

They lure Guy to an apartment owned by a family friend of Nicole. Mia drugs him, and they drown him in the bathtub. They wrap the body in a shower curtain and place it inside a wicker box.

On the way to the school, Nicole crashes Guy's car in a pileup on the interstate. The wicker box with the body goes unnoticed by authorities.

They arrive at the boys' school in the middle of the night and dump Guy's body into the unkempt swimming pool, staging his death as an accidental drowning. The act is carried out in darkness, and the pool's murky surface hides the crime.

Over several days the pool fails to bring Guy to the surface. Nicole has the pool drained, hoping to uncover evidence or mislead investigators, but the body remains missing.

The women discover photographs taken on the day of the murder, and fear that someone is blackmailing them. Paranoia grows as they try to identify the blackmailer and protect their secret.

Mia goes to view a John Doe in the sheriff's station, hoping for answers about Guy's fate. Instead, she meets Shirley Vogel, a retired officer turned private investigator, who offers to look into the disappearance.

Nicole resists Shirley's involvement, but the investigator quickly grows suspicious of the two women. Shirley's questions mount as she pieces together inconsistencies in their stories.

Shirley confronts Mia with the accident report from Guy's car and suggests he was en route to Pittsburgh on the day he disappeared. Mia becomes increasingly paranoid, convinced that Guy is alive and stalking them.

Two videographers filming a school event capture an image of Guy standing in a building window, fueling the impression that he is nearby. The sense of being watched heightens the tension between the women.

While investigating the school's basement, Shirley is attacked and knocked unconscious, intensifying the danger surrounding the trio. The incident raises the stakes for Mia and Nicole.

That night Mia finds Guy floating in her bathtub as if he rises from the water. She collapses, terrified and convinced she is having a heart attack.

Nicole laments that she had wanted to call it off, admitting the original plan. When Guy realizes Nicole is alive, he attacks the two women, knocking Nicole unconscious.

Mia flees downstairs as Guy tackles her near the pool, attempting to drown her. Nicole drives a garden rake into Guy's head, knocking him into the water; he pulls Nicole in, but Mia and Nicole drown him together.

The two women emerge from the pool, and Shirley confronts Mia, punching her and suggesting the self-defense rationale will help justify Guy's murder. Mia walks away from the pool in distress as Shirley watches Guy's body sink to the bottom.