Susan Ashworth, a lonely "cat lady" suffering from depression, attempts to commit suicide by swallowing an overdose of sleeping pills. She wakes up in an alternate world and meets the "Queen of Maggots", an enigmatic old lady who claims to understand Susan's distress. The Queen of Maggots grants Susan immortality and gives Susan a mission that she must fulfill to become happy: find five evil human beings called The Parasites, and kill them. Susan returns to the world of the living despite her wish to die forever. A young woman called Mitzi Hunt rents a room in Susan's apartment as part of her mission to take revenge on the man who encouraged her boyfriend to commit suicide. Susan and Mitzi slowly become close friends and look after each other as they try to further their goals.