The Enterprise responds to a Romulan ship that has been crippled in a catastrophic accident. La Forge and Ro beam over with an away team, but when they beam back, their transporter patterns are lost in an apparent transporter mishap. The two are declared dead, but they both wake up aboard the Enterprise — where they discover they are invisible to everyone around them and can walk through walls and other objects.
Are they ghosts trapped in purgatory? Please; this is Star Trek, where a ghost has never been encountered that cannot be ascribed to either technology or the presence of an alien life form. In this case, Geordi slowly realizes that they have been phase-shifted outside normal time and space because of experimental top-secret Romulan cloaking technology aboard the crippled Romulan ship (which was crippled because the technology blew up in their faces during testing). The Enterprise crew does not discover this in the course of their investigation, but La Forge and Ro, being invisible, are privy to more information.
Alongside the plot are some welcome character touches. Being the proverbial flies on the wall, Geordi and Ro get to listen in on conversations about themselves by shipmates who believe they are dead. A discussion in a shuttle between Riker and Worf has a certain amount of resonance — without being too earnest to get schmaltzy. Meanwhile, Ro — usually the firebrand — finds the whole situation oddly humbling. At first she believes she's dead, before Geordi is able to win her over with technobabble arguments.
For the most part, "The Next Phase" is content to be an entertaining but slight sci-fi adventure in which two invisible people must figure out how to make themselves visible — while also fending off an attack from an invisible Romulan who himself has been phase-shifted and wants to stop La Forge and Ro from revealing the Romulan cover-up of this experimental technology. Eventually we get a chase through the Enterprise where Ro and La Forge are running through walls and pushing the Romulan intruder through the bulkhead and out into space. Heh.
Of course, the nitpicker in me has to ask exactly what it is about this technology that makes it possible to pass through all objects except, of course, the floors. For some reason (that reason being plot convenience), the laws of physics are only selectively broken, so you can plant your feet on the floor instead of simply sinking straight through to who-knows-where. Maybe the technology is set to not affect the bottoms of one's feet. In which case, Geordi should've just gone around engineering kicking Data in the face instead of leading him around with mysterious energy signatures. So, yes, this is all goofy and absurd, but then many things in Star Trek are absurd, and in this case I'm not inclined to let absurdity get in the way of an amusing adventure yarn with cloaking technology that lets people walk through walls while teaching Ro Laren the virtue of humility.