Dr. Crusher falls in love with a Trill ambassador, with a twist.
Dr. Crusher falls in love with Odan, a Trill ambassador whom she meets during a diplomatic mission. When Odan is badly injured, it is revealed that he is actually a symbiont, a creature that lives inside a host body. A new host must be found, and Riker volunteers to carry the symbiont so that the mission can go on.
The episode poses a difficult question for Crusher, whether she can continue the relationship with a person who is now in a different body, and sex, and the answer is not simple. It was the first appearance of the Trill species, which later became central to Deep Space Nine. The story is a thoughtful exploration of identity, and it is a standout in the fourth season.