Goryo Daisuke is still unsure about which club to join. When he meets the three girl members of Mysterious Research Club while looking for the school's warehouse, he gets hypnotized to sign the registration form. Suzu who is obsessed with scary curses, Chiaki who only relies on her monstrous strength to perform dangerous magic tricks, and Kotone who can control people by hypnosis.

The sea in our world has been polluted. All sort of trash were thrown into the sea, making it unpleasant deep under the sea. One of the inhabitant of the sea, Ika Musume (Squid Girl) seeks to exact revenge on humanity for throwing trash into the sea. Clueless about the surface, she seeks to invade humanity alone, trying to rule over the world, only to find herself slaving away at one of the nearest beach-houses for breaking a side of its wall.