Star Trek: The Next Generation - Season 6 Episode 18 -

Stardate: 46682.4 - The crew evacuates the Enterprise when the ship needs to be cleaned of particles that have built up over the ship's hull. However, Picard accidentally stumbles over a terrorist group who plan on destroying the ship. He only has a short time to stop their attempts before the ship is swept by a beam of radiation to remove the particles that is lethal to all forms of life.