One player takes on the role of the escaped student. He is the Human player, and he is trying to rescue his friends and escape Innsmouth. By playing movement cards, he moves his pawn around the board to visit various locations, fights any Deep Ones there, then searches those locations for his friends and equipment to help in their escape.
The other players play groups of Deep ones. They are trying to prevent the Human player from escaping. The Deep one players take turns moving their pawns around the board trying to trap the Human. Certain locations on the board also allow them to draw cards, spawn additional Deep Ones or even summon a terrible Shoggoth.