Dr. Mora Pol, the Bajoran scientist who studied Odo when he was first discovered, arrives on Deep Space Nine and tells Odo that he has picked up DNA signatures similar to Odo's on a planet in the Gamma Quadrant. Mora, Odo, Dax, and Mora's assistant travel to the Gamma Quadrant and during the trip Mora tells embarrassing stories about Odo as many fathers would. Odo is offended.
Upon reaching the planet, the four of them beam to the surface, where they discover a tiny life form that may be a distant relative of Odo. Mora calls it "Odo Junior". Before they can return to the runabout, however, an earthquake releases volcanic gases into the air. The other three pass out, but Odo, unaffected because he does not have a respiratory system, beams them back to the runabout.
The life form is brought to the science lab on DS9, where O'Brien tries to identify it, with little success. However, it appears to be growing and he puts it in a containment field. That night, Kira contacts Sisko and informs him someone has broken into the science lab. Further investigation reveals that the life form appears to have broken out of its container and escaped through an air vent. O'Brien discovers that there was a power interruption and a temperature increase at the time of the incident. While searching a conduit, he hears a strange sound, and a minute later he discovers a puddle of goo. It is the life form's remains.
Bashir is studying the dead life form in the Infirmary that night when he is attacked by a strange creature. He fends it off and later discovers that the same power interruption and temperature rise occurred during the attack as when the life form broke out of its container. Dax discovers the DNA of the life form and the creature are different, but Mora decides to search for commonalities between the two. Dax tells him the computer will finish analyzing the DNA in a few hours, but Mora recognizes it and tells Odo that the traces left by the creature match his DNA.
Mora points out that the attacks occurred approximately sixteen hours apart, at times when Odo is due to revert to his gelatinous form to regenerate. Odo should have been in his pail during the attacks. They realize the gas on the planet in the Gamma Quadrant might have affected Odo somehow, turning him into some sort of creature during his regeneration period. Panicking, Odo begins to head for the Infirmary, but Mora claims Bashir will not understand and will treat Odo like a monster. He presses Odo to come back to Bajor with him, and as he does so, the increasingly agitated Odo again transforms into the creature.
Security picks up a power drain in Odo's office, but when they arrive to investigate they find nothing amiss. Having seen Odo's violent reaction to him, Mora explains to Sisko and Kira that he is what triggers the metamorphosis. The first time, he explains, the creature was trying to rescue the organism Mora had contained in the science lab; the second time, it appeared in the Infirmary, where Mora was a patient; and the third time, Odo and Mora were in the middle of a heated debate. He helps the crew lure the creature to the Promenade, where it is contained in a force field and reverts to normal. Once the effects of the gas are removed, Mora apologizes to Odo for causing him pain, and the two finally become close.