As with the real Dr. Bashir, the Changeling duplicate was portrayed by Alexander Siddig.
While attending a burn treatment conference on Meezan IV, the real Bashir was abducted and replaced by the changeling, who assumed his identity and returned to Deep Space Nine. The Bashir Changeling was very good in his role, to the point that no one realized that he was an imposter. While on the station he sabotaged the Federation efforts to collapse the wormhole. Because of his sabotage the beam that had been intended to collapse the entrance to the wormhole had the opposite effect, securing the wormhole to the point that it could not be destroyed by any technology the Federation had.
The Bashir Changeling then built a trilithium explosive that he intended to use on Bajor's sun to cause a supernova to wipe out the Bajoran system. This would have destroyed Bajor, DS9, and the fleets - Klingon, Romulan, and Starfleet - that had gathered at DS9 to fight the Dominion.
The Bashir Changeling was exposed when the real Dr. Bashir escaped from the interment camp where he was being held and alerted the station. The Defiant intercepted the runabout the Bashir Changeling had stolen and tractored it away from the sun before the Bashir Changeling could launch his explosive device. This caused the trilithium device to explode, killing the Bashir Changeling.
After the real Bashir returned to DS9, O'Brien remarked that he never realized that Bashir had been replaced. O'Brien teased that the infiltrator had been easy to get along with, and that he should've realized something was up.