A shuttle carrying Counselor Troi (and some poor guy named Ben that the episode doesn't care about because he doesn't also have awesome boobs) crashes on a planet. The away team beams down to rescue the survivors but encounters Armus (Mart McChesney), who initially resembles a miniature tar pit. He turns out to be an intelligent, albeit hopelessly embittered, being who can rise up and take humanoid form, and who has a voice that sounds like Megatron, only deeper and meaner. Maybe he's Unicron.
He also has the power to do ... well, whatever the plot requires him to do, including killing people at will and creating forcefields that prevent the beam-up of the shuttle survivors. Armus' biggest claim to fame is that he kills Tasha Yar, who dies a rather ignominious death, which is ignominious in no small part because of that goofy splotch on her cheek during the ER sequence — one of few TNG season-one moments to actually use hand-held cameras. (Things must really be bad when the hand-held cameras come out.)
You know, there really should be a "Skin of Evil" drinking game where you drink every time Armus rises up into humanoid form from his tar pit or descends back down, or every time he covers or uncovers the crashed shuttlecraft. Because it's a lot. If anyone sells that game, I expect royalties. Armus is occasionally amusing, simply because he's such an incredible bastard that you almost have to like him — or else hate Troi for trying to disarm him with her psychobabble. Come to think of it, maybe I'd just rather hate Troi in this episode.
The battle of wills (wits?) with Armus goes on for too long and gets too repetitive. (Did I mention that the scenes of Troi trying to counsel Armus really tried my patience?) The episode, at the very least, does not try to redeem Armus, and leaves him stranded and as unhappy as ever. Yar gets a holodeck funeral, which is well-intended, yes, but way too cloying and pushy. But what else would you expect from TNG?