Elio's family owns a successful holiday resort on the Spanish coast, but when multiple teenagers go missing in suspicious circumstances, bookings across the town dry up. Then Elio witnesses something strange - the sea parting for a teenage girl, before carefully enveloping her.

Unsurprisingly, no one on the island believes Elio. But a few days later, Sam, a British tourist, comes to find him. Her older sister Jess is one of the missing teens, and Sam thinks Elio's story could be true.

As they investigate, they too are pulled into the water and discover an underwater city ruled by selkie-like creatures. The beings are in desperate need of Elio and Sam's help, but why have they been taking teenagers, and can they really be trusted?