The One where Doc McCoy's ex is a salt luvin' monster
It's a bit daft this episode, but a strong central performance by Bones grounds this and gives the viewer a chance to connect to the characters beyond the silly story. Basically, Nancy is a shape shifter who hypnotises you and sucks your salt out of your body. She goes on to trick various crew members before Bones finally breaks the spell once Kirk is about to be sucked dry.
It's hard to take seriously , but it kind of works. There's too many "crewman see's their fantasy" moments, but the moments with McCoy are worthwhile and generally it's a fun watch. Bit slight though. Of course, famously, this was the actual first ever episode of Trek shown on TV. Why? Half the crew disappear in the next episode as they're shown out of order, not to mention uniforms etc. The mind boggles, but it spawned 700 plus episodes no real harm done i guess.