Hanne, Marthe and Julia have the vacation of a lifetime in the coastal village of Porto Sirento. Sun, sea, beach, having fun together, dancing and singing songs. But their vacation suddenly turns completely upside down, when a mysterious spell turns the girls into beautiful mermaids. If the girls do not want to stand out, they will have to avoid water, and of course that is easier said than done. Danger lurks everywhere. In the coastal village where the girls are enjoying their vacation, for some reason the inhabitants see mermaids as monsters. When the girls discover that after three days they will remain mermaids forever, a race against time begins to break the spell. If they don't succeed in time, Hanne, Marthe and Julia will be mermaids forever, and never again K3.