The premise for this episode is pretty out there, though you don't get to find out the most out-there part until near the end.
A shuttlecraft returning Counselor Troi to the Enterprise crashes on Vagra II, but something prevents the Enterprise from beaming them away from the crash site. While investigating this oddity, the away team encounter what appears to be a sentient oil spill that can move and change shape and apparently keep people from being beamed off the planet. This "Armus" kills Tasha Yar, just for laughs. Except that really, she's dead now. While the away team is back on the Enterprise, Troi communicates with the creature and finds out that it is a "skin of evil" abandoned on the planet by its creator-predecessors. Picard joins the away team this time, but there's a short hoola over Riker getting enveloped by Armus and almost killed. Once Riker is safe, Picard sends the rest of the away team back to the ship and convinces Armus to let him talk to Troi. Troi tells him what she learned, and Picard puts that together with findings from the ship's crew to figure out that angry Armus equals distracted Armus. So, he angers Armus and the Enterprise is able to beam him and Troi off the planet.
I can't decide if the pure evil thing is mindblowing or mindblowingly lame. Although its creation is highly improbable, having a creature who is pure evil is a nice break from the moral ambiguity needed to make the overall show feel realistic. Sure, we want realism, but we also want to be reassured that evil does exist and that the Enterprise can recognize and fight it.
Above all else, I thought the betrayal of evil acurate. Evil is powerful and pitiful at the same time. It's only objective is to hurt others and gain for itself. Evil is absolutely not the kind of person that you give a spaceship to no matter what they hold over your head. You leave evil behind if you can. Or, as Data put it, "Absolutely no redeeming qualities."
It is also nice to see Troi's talents used to tell us things we didn't already know. When an alien dignitary snarls at Picard on the viewscreen, we all get that he's angry. Troi doesn't need to tell us, unless she can give us a hint as to why. With Evil, she was able to discover that he had been abandoned. Then she used her training as a counselor to get more details out of it. Lastly, she told the captain what she knew and that helped him yell at evil one last time for the episode wrap up. Picard's performance angered evil, which weakened his concentration and let the Enterprise beam Troi and Picard out of there. Woohoo!
Oh, and I guess Tasha died. Seemed kind of abrupt and stupid. Also, my dad spoiled the surprise by saying at the beginning of the episode, "Isn't this the one where Tasha Yar dies?" Yeeeeep. It sure is.