S1E21 - "Meridian" (DS9 S3 E8)
What happens when you take an awesome sci-fi premise (planet phases between corporeal and non-corporeal state every 60 years) and turned it into a chemistry-less love story? Meridian! This Brigadoon-inspired episode is a certified snoozer, but is saved by a B-plot featuring Jeffrey Combs in his first Star Trek appearance as a bigtime creeper who wants a