“Rejoined” is episode six of season four of Star Trek: Deep Space Nine.
A group of Trill scientists are visiting DS9 to use the Defiant to do some research into the creation of artificial wormholes. However, this is not what the story is about.
One of the Trill scientists, Dr. Lenara Kahn (Susanna Thompson), is the host to a symbiont who was, in a former host, married to the host of Dax at that time. Both of the original hosts are now dead, of course – Dax’s being killed in a shuttle accident, and the wife of natural causes.
Under Trill law, new hosts are not allowed to reconnect with relationships of former hosts. Presumably intimate relationships, for Dax was a friend to Sisko in his former host, Curzon. The penalty for doing so is, eventually, death – the host and symbiont in question are exiled from the Trill homeworld, which means that when the host dies, the symbiont does too.
Unfortunately, both of them are still attracted, perhaps in love, with each other. Everyone is watching them closely to make sure they don’t rekindle their relationship.