Midsummer 1985 on Särsö begins in a sun-drenched idyll - until a young boy is found dead in the water. When Johannes and his friends arrive for the summer holidays, expectations are sky-high, but the island is marked by the mysterious drowning. During a forbidden excursion to the mythical island of Svärtan, Johannes and his friends make a discovery that changes everything.

Krilles' violent behavior scares the young people and Mattias begins to suspect that he is hiding something about Danne's death. Johannes, on the other hand, focuses all his attention on the mermaid and her strange powers. When he realizes how bad Helena is feeling, he lets her meet the mermaid - with unexpected consequences.