TNG : Season 1 Episode 23: Skin Of Evil

The episode is memorable for not only killing off a member of the main cast (one not even wearing a red shirt!) but also for the almost inconsequential way in which the death occurs.
The story sets up quite a bit that comes afterwards, and fortunately comes towards the end of the generally lackluster first season. As such, it’s quite obvious things have settled down and both cast and crew are more confident in what they are doing. I warn you though, this episode has a complete lack of any light-heartedness and is very much a dramatic presentation.

Plot:

The Enterprise receives a distress signal from a shuttlecraft returning Deanna Troi from a conference. They find that the shuttle has crashed on a desolate planet, Vagra II, and while they can find the life signs of Troi and the pilot, Lt. Ben Prieto (Raymond Forchion), they are unable to beam the two to the ship. An away team beams down and discovers an animated pool of a tar-like substance, a malevolent life form that calls itself Armus. When Lt. Yar attempts to approach the shuttle, Armus hurls her back with a psychokinetic blast, killing her instantly. The away team is brought back to the Enterprise but the damage to Yar is too great for Dr. Crusher (Gates McFadden) to repair, and they are unable to resuscitate her. A second away team is sent to the planet. Armus taunts the crew members and maintains his grasp of the shuttle. During this time, Troi has communicated with Armus and learned that it is a physical manifestation of evil from the bodies of an ancient race, abandoned on Vagra II.
The away team scans Armus' energy field, finding that when Armus engulfs the shuttle and speaks to Troi, expressing his remorse and pity, the field disrupting their transporter signals is weakened which could allow them to beam Troi and Prieto out of the shuttle. After Armus toys with Troi by completely engulfing Commander Riker (Jonathan Frakes), Captain Picard beams down to speak to Armus directly, sending the rest of the away team to the ship. Picard engages Armus in a heated discussion to discover its motive, which is to seek revenge on those that abandoned it on Vagra II. Armus is riled to a point where the energy field is dissipated enough, allowing for Picard, Troi and Prieto to be safely beamed back to the Enterprise, leaving Armus wailing in fury and alone once again. After destroying the shuttlecraft from orbit, Picard orders a permanent quarantine on Vagra II.
As they leave Vagra II, the crew holds a memorial service for Lt. Yar, with a pre-recorded simulation of Yar addressing each of the senior crew members telling them what they meant to her and what she learned from each of them. After the service, Data (Brent Spiner), who had previously become close to Yar, tells Picard that he is confused as to the purpose of the service. He says that his thoughts are not for Tasha, but for himself, because he can only think of how empty his life will be without her. He asks if he has missed the point, but Picard assures him he got it.