After a global apocalypse that has killed almost all humans, the only survivors are childhood friends Billy (Duplass), the former president of the United States of America, and Ray (Brown), a scientist and Billy's former adviser. Billy feels responsible for causing the apocalypse, and the two live in a dome built by Ray that protects them from the outside environment. They maintain a fishpond, but become distressed when the last remaining female fish dies, leaving behind only two male fish, one of which appears to be dying. Ray stays up late one night, and discovers the possibility that the fish are undergoing sequential hermaphroditism to continue breeding, and notices a bright green light outside the dome. The next morning, Ray tells Billy about his discovery, and Billy realizes that he is also undergoing sequential hermaphroditism and is gradually developing female sex organs.Billy is distraught over the changes to his body, and Ray is excited to study what is happening. Ray begins feeling uncomfortable around Billy after he becomes aroused when he touches Billy's breast. After the fish begin breeding, Billy reveals to Ray that he has begun to menstruate, and suggests that they try to have a child together. Ray rejects Billy, and refuses to speak to him. Ray admits to Billy that he is hesitant to procreate because he was raised with homophobic values, despite his efforts to be more progressive. Ray and Billy decide to have sex to try and impregnate Billy, initially with a sheet over Billy's body, which Ray eventually pulls off, and the two kiss. Billy becomes pregnant, and the two become much closer.A big storm hits the dome and breaks the windows, forcing Ray and Billy to break apart the fishpond to patch the broken panes. The next morning, the two are amazed to see some aurora borealis, indicating the sky is becoming less cloudy. Ray is distraught, as their supply of food is ending with the dead fish. Billy, using a story from Ray's eighth birthday, tells Ray to keep hope