Elysia is a Class M planet located in a sector of the galaxy that is not opened by the Federation for general transit, due to the lack of starship support facilities.

The planet appears to have been terraformed, as in its present configuration, it could not have developed naturally. Nearly the entire planet is a swamp with a noxious atmosphere, but there are ten anomalous areas, each with a breathable atmosphere, with a force field of unknown form keeping the swamp atmosphere from overwhelming the small habitats.

Nine of the habitats are inhabited by a humanoid race living in relatively primitive civilized societies. Equidistant to these habitats and in the center of them is a habitat that is nearly barren of vegetation, but has the planet's only mountain. Investigation shows that the mountain and the island are regarded as sacred by the Elysians. The Elysians are informed by their "gods" of visitors and the visitors are welcome to interact, no matter how strange-looking the visitors may appear to the Elysians.

Elysians are occasionally given permission to undertake a "quest", alone or occasionally in pairs, to cross the swamp to the island and pass tests, and if they pass, the "gods" grant a wish; the energy expended to grant the wish results in gravity surges that radiate deep into space from the planet.

On his own investigation of the island, Lieutenant Commander Data of the Federation Starfleet starship USS Enterprise-D joins with an Elysian woman, Thelia, to undertake a quest inside the mountain, and during it, Data suffers severe (though not irreparable) damage. When they succeed, Thelia's wish for her habitat to be joined with another nearby is granted. The "gods" grant Data his "fondest wish", and Data finds himself living as a human being, no longer an android. The "gods" later restore Data, and either it never happened, or time was altered starting from the moments before Data and Thelia met. (TNG novel: Metamorphosis)

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