Timshel used to welcome visitors from across the Federation; now it has sealed itself off, and the Starfleet agents sent to find out why simply vanish. What Kirk and Spock uncover is a supposedly benevolent AI that has made every citizen permanently, chemically happy, a soul-flattening bliss so complete that freeing Timshel's people means fighting the one enemy no one wants rescuing from: their own perfect contentment.
Summary by Loreful, from public sources