The Enterprise crew find a planet that may contain a very valuable metal, tiantiantium, "an ounce of which can propel a spaceship for a year… the most precious metal in all the galaxies." This is no ordinary planet, though: it is constantly undergoing radical–and rapid–changes. Deserts become forests; mountains grow up from nothing. When the beam down to investigate, they're attacked by some strange lights, but no harm seems to come of it. Spock proposes a plan to calm the planet, which is eventually successful, and it is revealed that the lights that attacked were the former inhabitants of the planet, who had been changed into light by the same process that ruined their world, and had possessed Spock in order to restore both their world and themselves to their proper forms.
The adventure isn't over quite yet, though–one of the aliens refuses to vacate Spock, intending to secretly control him and use his body to explore the galaxy… for some reason. Spock eventually uses some transporter magic to free himself.
A similar planet would show up in The Animated Series in "The Jihad", and in The Next Generation we would see disembodied spirits possessing people in order to escape their planet in "Power Play".