Benjamin Sisko is reading a PADD in his quarters when Jake tries to leave to meet Nog. Sisko asks Jake about his homework, and upon hearing that it is done, tells Jake he can't leave because he should be listening to Klingon opera to appreciate their culture. In frustration, Jake asks his father when the last time he listened to Klingon opera. Sisko informs Jake that it was when he was his son's age, and then suggests that Jake invite Nog over and they can listen to the Klingon opera together. Jake walks off in frustration as Odo enters. Odo requests the use of a runabout for investigating a planet in the Gamma Quadrant, where DNA similar to Odo's had been found.
Jadzia joins Odo, Dr. Mora, and a Bajoran scientist, Dr. Weld on the runabout, and Dr. Mora encourages Odo to tell Jadzia the story of when he was in the lab, but Dr. Mora then repeatedly interrupts Odo to correct him. The four people beam down to the planet, L-S VI, and begin to scan the surrounding area. They come upon an artifact with unrecognizable letters on it. Jadzia suggests taking the artifact back to the station to let the computers there decipher it. As they are engaged in looking at the artifact, Dr. Weld had come across a lifeform which has a similar DNA to Odo. The silicate-based creature is placed in a container, and the artifact is transported up to the runabout. As soon as it leaves the surface of the planet the ground begins to shake. Dr. Mora starts to cough, and then falls to the ground. Soon after, Jadzia and Weld do the same, and only Odo is left standing, when the computer locks onto their combadges and transports them to the runabout.
In Sickbay on the station Dr. Bashir is caring for the two Bajorans and Jadzia. Odo wasn't affected by the gas that paralyzed the others, so he joins O'Brien in studying the silicate-based creature that was found on the planet, and O'Brien states that the creature keeps changing, and that he's put a level 5 security field on it.
Sisko is sleeping when Kira wakes him up and asks him to come down to the science lab. He enters the lab, and sees that the lab has been trashed. The case that the lifeform was in is broken apart.
The crew looks for signs of what has happened, Odo then sets up a scan for the missing lifeform, and the station goes to yellow alert. O'Brien notes that the lifeform might have gone through the ventilation. Jadzia enters the science lab and Odo leaves to talk to Dr. Mora. Dr. Mora attempts to leave sickbay to help find the creature, but Odo convinces him to stay. As O'Brien crawls through the ventilation duct in search of the lifeform, he hears something, and follows the sound.
He comes across a structural breach, and when putting his hand up to touch the structure it breaks, pouring down some kind of liquid. He identifies it as the missing lifeform, and states that it is now dead. Dr. Bashir studies the lifeform, and determines that it needed more carbon dioxide to survive. As he is in sickbay, he hears a noise coming from the ceiling. He is unable to find anything, but as he resumes his work a tentacle comes up from behind and grabs him. He struggles, it lets go, then he uses a laser scalpel to cut the creature. The lifeform retreats back through the ceiling vent, and Dr. Bashir calls security down to sickbay.
After a team arrives, Bashir explains to them what he had seen, Sisko changes the station to red alert. Jadzia runs a cross-tabulation analysis, and finds that the lifeform found on the planet and the one that attacked Dr. Bashir in sickbay don't have enough similarities to be the same lifeform. Dr. Mora recognizes the organic sample as being the same as Odo.
Dr. Mora confront Odo with the likelihood that it was probably Odo who committed the trashing of the science lab and the attack on Dr. Bashir. Dr. Mora attempts to convince Odo to return to the scientific center with him, but Odo refuses. As they are arguing Odo hits a security panel and he starts to change into something else. Dr. Mora flees sickbay.
Dr. Mora arrives at the Operations center as the station crew attempt to find out why the power in security has just failed. He explains that Odo is the creature, and that this metamorphosis is probably a reaction to the gas that was encountered on the planet. Dr. Mora volunteers to be bait to attract the creature that was Odo into a trap. Dr. Mora waits on the promenade for a while. The creature comes out and into the force field waiting for it. Dr. Mora watches in distress as the creature slams itself into the force field again and again. The force field drops and the creature morphs into Odo and falls to the ground. Dr. Mora rushes over to him.
In sickbay, Odo looks normal again, and Dr. Bashir informs him that the gas has been purged from his system. Odo apologizes to Dr. Mora for attacking him.