Where is the line between hope and madness?

Stella only turned away for a second when her baby vanished. Though the little girl's body was never found, Alice was assumed to have drowned, but Stella has never been able to extinguish the hope that somewhere, somehow Alice is still alive.

Over twenty years have passed since that fateful day and Stella is a happily married, successful therapist. But when a new patient walks into her office, Stella's life starts to crumble.

The young woman introduces herself as Isabelle, but Stella is convinced she's Alice.