When William Riker from the Enterprise unexpectedly arrives on DS9 on vacation, he takes a liking to Kira, who has also been ordered to take some time off. However, when Kira gives Will a tour of the Defiant, it becomes clear that there's more to Will's vacation than meets the eye.

After a transporter malfunction aboard the Defiant, Sisko, Bashir, and Dax find themselves thrown back three hundred years into Earth's dark past without any means of getting home.