Quark's selling body parts again. Odo points out that the Ferengi he's selling body parts from is still alive. Quark pleads ignorance. Dr. Mora Pol, the scientist who originally studied Odo, turns up.
Jake is studying Klingon Opera. Poor Jake. Odo asks to use a runabout because Dr Mora wants to investigate something in the Gamma Quadrant - some DNA patterns like Odo have been located on a planet there.
Dax discusses with Pol and Odo how Odo first made contact with the Bajorans. It's rude to talk about someone in the third person when they're sitting right next to you. They beam down to the planet and find ruins including an interesting pillar with markings on. Dax suggests they take it back with them. (It could fit on the runabout?!) They find some living matter which might be another changeling. After they beam the pillar up, the ground starts shaking and they hurriedly beam up.
Unfortunately some gas has stunned Dax and the Bajorans for the time being, so Bashir puts them on support when they get back to the station. Meanwhile, the sample is growing in size at an alarming rate. The next morning, it has broken out of its containment field
Sisko goes to yellow alert and they start sensor sweeps of the station to find the life form. Dax seems to have woken up, and takes another look at the pillar which they brought back. O'Brien tracks the life form through the conduits and finds it dead. Later on, something attacks Bashir in medical. It appears the life form isn't dead at all. Sisko goes to red alert.
Dr Mora is amused by the other crew members addressing Odo as "Constable", and says he's proud of the Changeling. He explains to Dax that he and Odo originally didn't part on friendly terms, but he's impressed by how far Odo's come. Dax runs an analysis on the residue left by the creature that attacked Bashir, but Mora recognises it as a sample from Odo, and sods off to hell him.
Odo says he was in his bucket at the time of both attacks, and they wonder if the gas on the planet didn't affect him in some way too. After Mora harasses Odo, he starts melting and turning into whatever. Mora backs out of security and reports to Ops to relate what he knows.
They set about trying to capture Odo. They manage to lure him onto the Promenade and capture him in a force field. Yo, that's some slimy CGI. Odo reverts to humanoid form and goes into therapy. Bashir and Mora eliminate all traces of the gas from him, and Odo makes up with Mora. Awwwww. The end!