Star Trek: Deep Space Nine, season 3, episode 8
Meridian
Summary
The Defiant crew discover the planet Meridian that phases between dimensions, appearing as corporeal for only a few days every 60 years then phasing into a state of pure energy. Dax falls in love with a native, Deral, and opts to join him and his people when the planet phases back into non-corporeal form.
Summary from Wikipedia, CC BY-SA 4.0.
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