Crusher falls head over heels in love with a visiting Trill ambassador named Odan (Franc Luz), who is assigned to negotiate a solution to an alien dispute that is threatening to escalate into war, pursuant to the Two Warring Factions standby oft employed by TNG. The Federation knows so little about Trill society at this point they don't even know they are a joined species.
So Crusher is shocked and saddened when Odan is critically injured in an attack on a shuttlecraft and it turns out the symbiont slug inside him (not even referred to as a symbiont here) is actually "Odan" and the external body is just a host. The symbiont is joined with Riker as an emergency to keep Odan alive until a replacement Trill host is sent. It's interesting to look back at "The Host" and realize how much the Trill backstory and rules evolved after DS9 came around. The host here is depicted as more of an empty shell rather than a fully participating half of a joined whole, which begs the question of where Riker's mind goes while Odan is joined with him.
Odan's negotiations with the Two Warring Factions are pure MacGuffin, and I frankly don't care. But as a romance, "The Host" works for all the reasons "Half a Life" fails. First of all, we have an actual spark of chemistry between the leads. "Half a Life" was labored and stolid, whereas "The Host" shows evidence of actual passion and emotional risk. Second, we have an alien element to the story that actually enhances the storyline rather than detracting from it. "Half a Life" was about people who kill themselves at 60, which is so arbitrary as to make it impossible to become emotionally invested in the premise. "The Host," on the other hand, asks an interesting question: What is it that defines us in the eyes of a lover? How important is the physical component of love, when you know someone by touch and by sight and by the sound of their voice? If the same person you knew had a different external package, would they be the same person?
These questions put "Doctor Beverly" through the ringer in fairly interesting fashion — although I think the story would've worked better if a regular character had not been the emergency host. Riker's role as host merely complicates matters (is it ethical for Odan and Crusher to pursue the relationship while Odan is in Riker's body?) and provides a distraction from the true heart of the story, which is: How far does love transcend our physical presence?