Arjin, a Trill initiate trying to qualify for joining with a symbiont, is nervous that he must study under Dax, having heard horror stories about the tough-as-nails Curzon and other hard-to-impress Dax hosts. However, when he meets Jadzia Dax in Quark's, it is he who is taken aback by her casual, lighthearted demeanor. Later, Dax and Arjin take a Runabout into the Gamma Quadrant, and she explains that he doesn't have to worry about impressing her. Suddenly, the Runabout gets snagged on a small mass of protoplasm in a subspace pocket — an unidentifiable mass.
The crippled Runabout returns to the station with the matter still attached. While O'Brien prepares a containment chamber for the mass in the Science Lab, Dax takes Arjin to dinner, where she expresses worry that he only seems concerned with meeting other people's expectations, and observes that Arjin has few aspirations or goals beyond being joined. She later reveals to Sisko her reluctance in confronting Arjin about her impressions because Curzon Dax, who trained Jadzia, was hard on her — to the point of giving her an unfavorable recommendation. Sisko insists that it is her job to be honest with Arjin in order to help him learn.
O'Brien discovers that Cardassian voles — small, rodent-like creatures that have infested the station — shorted out the containment field that held Dax's protoplasm, which is now glowing. While she and Arjin study it, Dax finally tells him her concerns, which causes Arjin to angrily leave the room, thinking the worst. Later, Dax reports that the matter is actually a rapidly-expanding proto-universe, which, as it grows, is displacing universe. Sisko reluctantly decides that their only option is to destroy it, until Dax finds indications of life within it.
Sisko is left with a tough dilemma — deciding how to protect the station without destroying the matter and possibly murdering an entire civilization. While he ponders his choices, Dax locates Arjin and explains how the shy Jadzia made it through the program despite Curzon — and finding a newfound inner strength in the process. As she urges Arjin to do the same, Sisko arrives with his decision — he has opted to try and take the matter back through the wormhole. Dax invites Arjin to pilot the Runabout with her, and he agrees.
O'Brien creates the strongest containment field he can for the matter, but it begins to collapse once the Runabout encounters vertiron nodes in the wormhole. Dax orders Arjin — an expert pilot — to navigate through the veritable minefield of nodes, despite his reluctance. The field then collapses, leaving no margin for error, but Arjin succeeds in getting through the wormhole, returning the developing universe to its subspace pocket, and finally winning Dax's respect.