Puck Ekstedt, doctoral student in literature, is invited by her supervisor Rutger to spend Midsummer with him and his wife at their summer house on an isolated island. When he reveals that the young historian Einar Bure will be there too, Puck gladly accepts. When she arrives to the Midsummer party on the island, she finds a bunch of young people, whose lives are entwined since their student days. She quickly notices a web of tensions between them, both erotic and distressing. These tensions grow, when Rutger’s former fiance Marianne Wallman unexpectedly turns up with her girlfriend Viveka Stensson. Next morning Puck stumbles upon Marianne’s dead body in the wood. Puck and Einar go with the motor boat across the lake to call for Einar’s friend Christer Wijk, chief of the national homicide investigation team. When Christer arrives to the island, the boat’s motor breaks down. There is no other boat, and no telephone, so now all of them are isolated together on the island. The murderer must be one of the Midsummer celebrators, because there is no one else here. Christer’s suspicions are focused on Georg Malm, a former lover to Marianne, but the next day he is found murdered as well.