Passagier 23.

It's been five years Martin's family vanished without a trace from a cruise ship. Ever since, the police psychologist is tortured by the thought of his wife having committed a murder-suicide, taking their son with her. He is the only one not willing to believe this. Suddenly, a new lead takes him back aboard the very same luxury cruiser, where once again, a mother and her child have vanished. But this time, the girl reappears - holding a teddy bear that used to belong to Martin's son. Martin soon learns that things are handled differently on the open sea. Not to disturb his passengers' enjoyment of the lavish luxuries of their cruise, the rich owner of the cruiser wants to keep the child out of sight and hides her deep under deck in the quarantine bay. Here, Martin tries to get the traumatized girl to talk to him. But even deeper in the dark underbelly of the ship there is something lurking. And all signs point to a serial killer, just waiting to strike again.