Guinea Pigs

The Captain takes Vector 3, alone, into Breen space to retrieve a Trill multi-spacial shielding prototype. Things are turned around when the Commander and the Doctor join him, and find a dieing symbiont, at the tech site. A decision is made quickly, forcing the Doctor to join the symbiont with the Night host temporarily until they can get it to safety.

Links
Episode 15 - "Guinea Pigs"