Pilgrim of Eternity

Investigating unexplained energy drains at Federation power stations, the Enterprise encounters an alien artefact that starts siphoning power from the ship itself. The crew destroys it, but Spock detects life signs aboard, and the survivor beamed over is Apollo, the Greek god Kirk's crew met two years earlier. Now frail and aged, he insists he no longer craves worship. Once restored in sickbay, though, his old hunger returns, and he turns his psychic power on Kirk and others until the ship's new counsellor, Dr Elise McKennah, fires a phaser to protect the captain. Chastened, Apollo asks McCoy to remove the organ that turns worship into life energy, and the surgery leaves him weakened. Meanwhile, debris from the wreck causes a fatal accident during hull repairs, and Scotty's fix ends with Uhura killed by a massive shock. Ignoring Spock's warning, Apollo spends his strength to bring her back, collapses, then revives stronger than before. Kirk realises that sacrifice, not adoration, may be the real source of his power. The crew leaves Apollo on a world of humble, medieval-level natives where he can serve rather than rule, and a final glimpse a year later shows him helping the people and looking younger.