Ayase Momo is a girl that has psychic powers and believes in ghosts. Her psychic powers were activated when she was kidnapped by aliens that are trying to repopulate through her.
She makes a bet with Takakura (who believes in UFOs) to see whether ghosts or UFOs are real.
This bet results in her getting kidnapped by aliens and Takakura getting cursed by a ghost.
Ayase lives with her grandma who is a psychic in a shrine. Ayase didn't believe her grandma had an ability initially, but she later apologized to her when she found out that her powers were real.
Ayase gave the nickname "Okarun" to Takakura because she can't accept the fact that he shares the exact same name as her favorite actor "Takakura Ken."