During the Nazi occupation of a Czech city civilians are being rounded up on the slightest of pretexts and shot. One day three high school boys who crack jokes about a recently deceased "hero of the Reich" are pulled out of school by the Gestapo.

The little mermaid rescues a prince from drowning and falls in love with him. To be with him, she makes a deal with the evil sorceress: her beautiful voice against a life on land.