O'Brien's wife is possessed by an entity and he is told to help.
Keiko O'Brien returns from Bajor and reveals to Miles that an alien entity has taken over her body. It forces him to carry out a series of secret modifications to the station, threatening to kill Keiko and Molly if he tells anyone. O'Brien concludes the being is a Pah-wraith, one of the legendary enemies of the Prophets.
O'Brien recruits Rom to help, and the two eventually realize the changes are designed to fire a beam into the wormhole that would destroy the Prophets. Instead, O'Brien turns the beam on a runabout where the possessed Keiko waits, driving out the Pah-wraith without harming her. Rom is rewarded with a move to the day shift.