A young Trill man arrives on the station. He is to study with Jadzia Dax, and the prospect makes him ill-at-ease and defensive. He has heard rumors that Dax hosts have been very harsh mentors, especially Curzon. He hopes to please Jadzia, since he must get a recommendation from her if the Trill joining committee is to accept him as a host. When Arjin meets Jadzia Dax he is surprised to find her so laid-back and free-spirited.
Arjin's first job is to accompany Dax through the Wormhole in a runabout. An accident occurs and the two Trills bring the runabout back to the station damaged. There is a strange mass of matter attached to it. Chief O'Brien removes the mass and contains it securely for further study.
Meanwhile, Dax has been getting to know Arjin. He seems aloof but preoccupied with what other people think of him, and may be applying for a symbiont to please his father and others. He seems to have few interests or goals. Dax is nervous about confronting Arjin about his motivations. When she confides in her friend Sisko, he reminds her about her time training under the notorious Curzon. He was very hard on her, and challenged her confidence in herself.
Trouble starts when the unidentified mass in the lab is released from its containment field. O'Brien is annoyed to find that the station has a pest control problem. There is an infestation of Cardassian voles, and they are beginning to short out crucial systems, including the power source for the science lab containment fields. When Dax and Arjin find the matter, it has begun to emit an eerie glow. They run calculations and realize that the matter may actually be a tiny universe. It is expanding quickly and will soon pose a danger to the station. The crew prepares to destroy the universe, but are halted when Dax finds that it holds signs of life.
As Sisko ponders the matter, Dax speaks to Arjin about his performance. She reveals that Curzon's high expectations for her actually helped her become stronger and more ready to become a host. Before she was joined to Dax, Jadzia was shy and sensitive, and Curzon often reduced her to tears. But in the end, she got her symbiont, and she encourages Arjin to strive for more.
Sisko has weighed the options and has decided the proto-universe must be removed from Deep Space Nine. Dax and Arjin board a runabout to ferry the universe back through the Wormhole. She allows Arjin to take the helm, a challenge that he accepts.
As they rush through the Wormhole, the expanding proto-universe becomes unstable, making the runabout difficult to fly. Arjin can barely control it, and Dax offers to take over. He resolves to do the job himself, and as he safely brings them out of danger and releases the little universe out into space, he proves himself and earns his positive recommendation from Jadzia Dax.