When the entire universe seems to "blink," the Enterprise suspects invasion but finds only Lazarus, a man who seems to alternate between two different extremes of nature. In time, "he" is found to be two twinned beings locked in eternal struggle between their matter and anti-matter universes — a rift Kirk must help seal.
Mike and Robbie each have dates the same night. Due to a mix up in names, Mike picks up Robbie's young date (Trudi Ames), and Robbie picks up an older woman (Carole Costello), meant for Mike.
After learning she was adopted, an heiress seeks her biological mother; guest Eileen Brennan.