The story is based on traditional Slavic folklore about the rusalki, river-dwelling mermaids said to be "born" from the unhappy souls of young women who had committed suicide by drowning—usually after being mistreated by a man.
The Russian "mermaid" is, for this reason, a dangerous creature more akin to the Greek sirens than to the American archetype such as Walt Disney Animation Studios' cute and lovable Ariel.
As springtime begins to break up the ice on a frozen river, a handsome young novice monk rescues a naked girl who has apparently fallen into the water.
The young monk's elderly master comes to understand that the rusalka is actually the drowned spirit of a girlfriend he himself had loved and betrayed as a youth, and takes steps to protect his naive apprentice from her supernatural revenge.
But when the mermaid recognizes the old monk as the man who had once jilted her, it may not be the young apprentice who is in mortal danger.