A man claims to be hunting a madman, and the universe itself seems to wobble at the edges.
The Enterprise detects a strange disruption in space and time and rescues a man named Lazarus, who says he is hunting a dangerous madman. Soon the universe flickers, and it becomes clear there are two versions of him, one in each of two universes that must not meet.
The episode uses a confusing set of effects to suggest antimatter and parallel realities. It is often ranked low by fans, but it introduces the concept of a corridor between universes, an idea that Trek would revisit with greater polish later in the franchise.