A Romulan delegation arrives to study reports on the Dominion, a privilege granted in exchange for the use of a cloaking device aboard the Defiant. Meanwhile, O'Brien, at Quark's riding out a mild case of radiation poisoning from an accident, is suddenly transported to another part of the bar, where he sees himself having a conversation with Quark. He just as suddenly returns to where he was, then falls over in pain. Bashir blames the radiation and assures O'Brien that hallucinations are a normal side effect. But when O'Brien returns to the bar five hours later, he has the same conversation with Quark that he "hallucinated." Dax theorizes that O'Brien's exposure to radiation could have caused him to temporarily shift five hours forward in time. Then, O'Brien experiences another timeshift — into the middle of a brawl in Quark's between the Romulans and some Klingons.
Bashir examines O'Brien in the present and determines that the timeshifts pose a serious threat to his life. Five hours later, O'Brien returns to Quark's to wait for the brawl, and the Klingons soon arrive and exchange blows with the Romulans. O'Brien timeshifts again, and this time sees himself in a corridor. He watches as his future self opens a wall panel and is killed by a blast from a hidden phaser.
After shifting back, O'Brien leads Sisko and Odo to the wall panel, but they find nothing. They assume that a weapon will soon be placed within it over the next five hours. While Bashir works to rid O'Brien's body of the residual radiation, Kira reports that she has moved the Romulans to the quarters adjacent to the wall panel. When Odo discovers that someone has put a surveillance device behind the panel, he suspects the Klingons are spying on the Romulans. O'Brien then shifts five hours into the future again — and sees his dead body in the Infirmary.
In the future, Bashir reveals to O'Brien how the radiation killed him. When O'Brien shifts back, the doctor is able to use this knowledge to save him. Meanwhile, Dax locates the cause of the shifts, which is a quantum singularity orbiting the space station. It periodically affects the radiation in O'Brien's body and triggers the timeshifts. O'Brien then shifts again into the future and finds himself aboard a crowded Runabout in the midst of an emergency evacuation — during which he watches as the station explodes and the wormhole collapses.
Hoping to learn how both the station and the wormhole are destroyed, O'Brien uses a device to flood himself with radiation in order to travel to the future again. Once there, he and his future self join forces, discovering that the quantum singularity is inside a cloaked Romulan Warbird, which becomes visible and attacks the station. While the timeshifting O'Brien becomes dangerously sick from the radiation, his future self is forced to take the device and return to the other's time period, where he tells Sisko what will happen. The commander deduces that the Romulans plan to eliminate the threat posed by the Dominion, making the station's destruction look like an accident from the wormhole collapse. Sisko expels the Romulans, and the future O'Brien must get used to living a few hours in his past.