A shuttle crash leaves Troi stranded with a vengeful oily entity.
A shuttlecraft carrying Counselor Troi crashes on Vagra II and is held by a malevolent being called Armus, a living pool of tar-like oil formed from the evil left behind by an ancient race. Armus delights in tormenting the crew, and the away team cannot easily rescue the survivors. When Tasha Yar steps in, she is killed.
The death of the security chief was a shock, as it was unusual for a major cast member to be killed in a television series. The episode ends with a memorial service in which she leaves a holographic message for her friends. The story leaves a question about how the crew responds to loss, and is often cited as a turning point for the series.