While scouting what seems to be a planet of "paradise," the landing party encounters trouble that results in several of Kirk's men (read: red-shirts) being killed. The party subsequently stumbles upon a primitive but peaceful civilization that lives to serve Vaal, a machine worshiped by the planet's inhabitants as a god.
Where did Vaal come from and why was he put there? Beats me. "The Apple" doesn't seem to know or care, either. This is a lackluster analysis of a stagnant, naive society whose existence is dictated by a machine. For once, however, Kirk doesn't make this machine blow up by offering it a circular argument; instead he has Scotty open fire on Vaal with the phasers.
The only real aspect of any real interest in "The Apple" is the wonderful (but regrettably brief) Spock/McCoy debate on the Prime Directive, proving that Trekkian polemics can be interesting. Other than those few short scenes, the episode is basically a bore. Everything from the ship being put in routine jeopardy to a half-baked theme about physical love, "The Apple" just doesn't work.