Timeship of Lazarus
"The Alternative Factor"
After a weird "winking out" phenomomenon, where the Universe seems to disappear for a moment, the Enterprise picks up a strange being called Lazarus who claims to be hunting a demon creature. This adversary proves to be another Lazarus; both are from parallel universes, one being negative, the other positive. If they should meet, the cosmos will be annihilated! Kirk finally destroys Lazarus' timeship, thus preserving the galaxy.
