The Assignment

Keiko returns from a trip to Bajor, but she is no longer herself. She tells Miles that an alien entity has taken over her body, and that his family will pay unless he does exactly as it says. Unable to confide in anyone, O'Brien starts making a series of mysterious modifications to the station, enlisting Rom as his assistant. As the work draws suspicion, O'Brien is forced to let Rom take the blame, and Rom is detained but refuses to talk. Together the two work out that the changes will create a beam designed to kill the Prophets, and that Keiko's captor is a Pah-wraith, their ancient enemy. After overpowering Odo, O'Brien completes the work, then lures the entity onto a runabout and fires the beam there instead, destroying the Pah-wraith and leaving Keiko unharmed. To thank him, O'Brien moves Rom to the day shift.