Odo's mentor arrives on Deep Space Nine intent on resuming his search for Odo's true origin.
Dr. Mora Pol, the Bajoran scientist who studied Odo after he was discovered, arrives on Deep Space Nine with the announcement that he may have a clue to Odo's origins. The scientist's presence makes Odo, who did not relish his life as a "living experiment," somewhat uncomfortable, but he cannot resist the prospect of possibly finding others like him. With this in mind, he asks Sisko for a Runabout to take to the Gamma Quadrant, where Mora has discovered some unique DNA patterns that resemble Odo's. Odo, Mora, Dax, and another scientist, Dr. Weld, head for the Gamma Quadrant and transport to the surface of the planet where the DNA was discovered. There, the group explores some stone ruins until Weld discovers a tiny lifeform that may be related to Odo. They prepare to transport back to the Runabout, but at that moment, the planet is rocked by tremors and clouds of a volcanic gas are released. Mora, Dax, and Weld are all incapacitated by the fumes, but Odo is seemingly unaffected and transports them all back to the ship.
Odo pilots the Runabout back to the station and the others are transported to the Infirmary. Later, O'Brien, filling in for the ailing Dax, attempts to classify the lifeform the group retrieved, and tells Odo that he is having a hard time because the organism keeps changing and reproducing. That night, Kira summons Sisko with some disturbing news - someone has ransacked and almost completely demolished the Science Lab.
The officers realize the lifeform is missing, and since there is no sign of a break-in, O'Brien theorizes that the lifeform itself caused the damage. Sisko orders a sweep of the station to search for it, while O'Brien searches the conduits. Communicating with Sisko and Odo, O'Brien learns that there was a power interruption and a temperature increase in the Science Lab at the time of the "disturbance. " Suddenly, the dead remains of the lifeform, which appears to have grown, fall on O'Brien. Bashir studies the inert material later that evening, and is attacked by a strange creature. He is able to fend off the creature, but it disappears into the night.
The officers assemble to investigate, then Mora accompanies Odo to the lab, where a recovered Dax discovers the lifeform from the planet is not the same creature that is wreaking havoc on the station. Mora looks at the data and realizes that instead, the violent creature's DNA matches Odo's. Without telling Dax, he tracks Odo down and breaks the news.
Odo declares that he was in his pail during the attacks and is upset by Mora's accusations that he may inadvertently be responsible. However, Odo realizes that the gas from the Gamma Quadrant planet may be to blame, sees through the doctor's concern, and firmly tells Mora that he will not leave with him and become his subject again. At that moment, Odo morphs into the creature. Sisko orders the power shut off and Mora lures the creature to the Promenade, where it is captured and morphs back into Odo. Odo is then taken to the Infirmary and cleansed of the gas, and Mora pays him a visit. The doctor apologizes for the pain he caused his former subject, and Odo, having forgiven Mora, finally begins to form a true bond with his mentor.
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