After being abandoned as a child, Jack ventures to remote New Zealand to attend the funeral of his estranged mother and there meets her grieving widow, Jill. His search for answers becomes dangerous when his mother’s ghost returns to inhabit both Jack and Jill, using each of their bodies to speak to the other, and instigating a life-threatening nocturnal dance between the three of them.
When Jack is possessed by his mother, she uses his body to have sex with Jill. She does this with no consent from Jack. When Jack wakes up and is beside Jill in bed, he breaks down realizing what has happened.
Mild spoilers: Jack is held underwater when he's being bathed by Jill (possessed by his mom). He is also held underwater by Jill at the end of the movie, when he falls through the ice.