“The Alternate” is episode twelve of season two of Star Trek: Deep Space Nine.
The Bajoran scientist, Dr. Mora Pol, who was originally assigned to Odo when he was first discovered – back when they didn’t know what he was, or even if he was sentient or alive – visits DS9. He wants to borrow a runabout, as a Bajoran probe in the Gamma Quadrant has discovered traces on a planet in a system close to the exit of the wormhole that resemble only one thing discovered so far – Odo himself.
Odo, Dax, Mora Pol and another scientist take a runabout through the wormhole to the planet. The planet is suffering a lot of seismic activity. They beam down to ruins of some sort of civilisation, and take away an unusual obelisk and a weird lifeform, but when the obelisk is beamed aboard the runabout, there is an increase in seismic activity and a gas is emitted.
Back on the station, the Bajorans are suffering the effects of the gas, but Odo seems fine, as does Dax. The lifeform they recovered apparently breaks out of the science lab, and then dies, but there is something else with shapeshifting abilities on the station.