Sisko and the crew travel back in time to a pivotal moment in the history of the original Starship Enterprise. The U.S.S. Defiant is returning from Cardassian space with the Bajoran Orb of Time. A Klingon named Arne Darvin, surgically altered to look human, has come aboard as a passenger. He uses the Orb to send the U.S.S. Defiant more than one hundred years into the past--near the original U.S.S. Enterprise as it orbited Deep Space Station K-7. Records reveal Darvin was a spy then, and that he was caught having poisoned a shipment of grain on the station. Darvin is out to change history, possibly by killing Captain James T. Kirk, who originally exposed him as the spy.