So the Enterprise is going to rendezvous with Troi who's coming back from somewhere or other in a shuttle. Yar is going to take part in a martial arts tournament, and asks Worf for some help practicing. Then they receive a distress call from the shuttle, but can't go to warp because the chief engineer has removed the dilithium crystals to realign them. Great timing there, buddy. Y'know, I'd have expected dilithium crystals to be more regular in shape, rather than looking like someone's sculpture of a mountain. The chief engineer finally gets the crystal back in and Enterprise proceeds to the crash planet at warp.
They can't beam up the shuttle survivors because something is interfering with their sensors, so Riker beams down with an away team. That's an odd-looking shuttle. The away team encounters a black substance which moves to block their path to the shuttle. Data says there's insufficient information about it, but it changes into humanoid form and starts talking, introducing itself as Armus. When Yar goes towards the shuttle, it zaps her. Riker and Data try unsuccessfully to phaser it.
Crusher reports Yar is dead. Despite getting her to sick bay, she fails to respond to treatment.
Back in the shuttle, Troi comes around and has a discussion with Armus about the death. Great, psychoanalysis for alien life forms.
Picard appoints Worf acting head of security, and they discuss what to do about Armus, resulting in Riker beaming down with another away team. Troi analysis Armus some more, then Riker has a chat and they manage to persuade him to let Troi contact the away team.
Armus explains to Troi that the race that used to live on the planet perfected a way to purge negative emotions, creating him, and then sodded off. After she says she pitied him, Armus returns to oil form and pulls in Riker.
On the ship, Worf and Wesley are charting the creature's energy levels and have worked out a pattern. Picard beams down to chat to Armus and he proceeds to play games with them some more before getting bored and spitting out Riker. Picard has the rest of the away team beamed up and persuades Armus to let him into the shuttle and talk to Troi who reveals Armus' energy lowers when she psychoanalyses him. Picard pops back out and does some psychiatry. Once its sufficiently pissed off, Worf beams everyone out. They phaser the shuttle from orbit and declare the planet off limits.
Psychiatry saves the day! Take that, Scientology!
Then they have a funeral for Tasha on the holodeck, and HoloTasha gives a speech. You'd think the holodeck could manifest a solid HoloTasha. Teh end.
Despite being overshadowed a bit by the fact that Tasha is offed, I thought this was a pretty good story. The concept of living evil is interesting, though the story could have done with some more action...