The title pretty much gives it away. However, when I was young (like 12 or 13) and viewing the episode for the first time, I totally didn't see it coming.
Fifty years after its original broadcast, "Third From the Sun" still manages to be completely entertaining, despite what amounts to an obvious surprise ending. Fritz Weaver, Joe Maross, and Edward Andrews star (all three of them, incidentally, would appear in later TZ episodes) in an episode written by Rod Serling, based on a Richard Matheson short story (this time around, Serling is much more faithful to the source material than the last Matheson adaptation, detailed here). Richard L. Bare directs. The underscore is made up almost entirely of selections from Bernard Herrmann's "Outer Space Suite," a collection of musical cues which would be utilized in several future Twilight Zone episodes.
The episode features ingenious cinematography work from DOP Harry Wild, who fills the frame with odd angles and unconventional camera set-ups. This in and of itself is strange, since George T. Clemens was the DOP on almost every TZ episode throughout its five year run. I can't find anything documenting why Clemens didn't work on this episode... maybe he was sick that week? I dunno.
This episode also marks the first appearance of the flying saucer from Forbidden Planet. This particular prop would go on to appear in many Twilight Zone episodes, most notably season four's "Death Ship" (one of my all-time favorites).