Howard Silk, a gentle, quiet office worker, has been working for a Berlin-based United Nations agency, the Office of Interchange (OI), for thirty years. His position is too low for him to be told what his work—exchanging apparently nonsensical messages—really involves. The OI oversees a checkpoint below its headquarters between parallel Earths (the "Alpha" and "Prime" worlds). The parallel Earths were created in 1987 during an experiment by East Germany when only a scientist named Yanek was on-site. The "Alpha world" Yanek met his "Prime world" counterpart, and they soon began studying how the initially identical Earths diverge.