As well as leading the Dominion, the Founders also act as its eyes and ears by using their shapeshifting abilities to infiltrate the facilities and ships of their enemies. In the years leading up to the war between the Dominion and the Federation, numerous changeling operatives had been planted within every major power in the Alpha Quadrant.
Since the Founders do not differentiate from each other and have no individual names, the changelings listed below are named after the identities they have stolen.
Theodore KrajenskyPortrayed by Lawrence Pressman. Krajensky was a Federation ambassador who was replaced by a changeling before taking an extended vacation to Risa. The changeling that took his form appeared in the DS9 episode "The Adversary" and assigned Benjamin Sisko (who had just been promoted to captain) to investigate a coup d'etat on the Tzenkethi homeworld. The "ambassador" joined the Defiant crew on the mission and went about sabotaging the ship by planting devices all over the vessel that took possession of key systems, locking the ship on course and arming it for battle. The changeling's mission was to trigger a war between the Tzenkethi and the Federation, destabilising the Alpha Quadrant and making it ripe for Dominion conquest. The crew were able to discover the changeling's deception and the changeling escaped into the Defiant's Jefferies tubes, leading to a shipwide manhunt. In the end, Chief O'Brien was able to win back control of the ship as Odo and the Founder (who took Odo's usual form to confuse O'Brien) were caught in a stand-off. The shielding around the warp core had dropped when O'Brien eliminated the Founder's sabotage devices and as Odo grappled with the changeling, the pair of them stumbled against the engine core that was hot with radiation. The Founder fell right against the engine core and took a massive surge of energy that left it gravely wounded. As it lay half-melted on the deck, the changeling whispered to Odo "You're too late, we're everywhere", indicating that the Dominion had infiltrated every major power in the Alpha Quadrant. As the changeling died, his gelatinous form turned to a pile of black ash.
General MartokA high-ranking commander in the Klingon Defence Force, General Martok (portrayed by J.G. Hertzler) was replaced by a changeling while hunting saber-bear on Kang's Summit on Qo'noS. First appearing DS9's fourth season premiere "The Way Of The Warrior", the Martok changeling destabilised the peace in the Alpha Quadrant by fooling the Klingon High Council into believing that the Dominion had engineered a coup on Cardassia Prime. The Klingon Empire waged war against the Cardassian Union, which led directly to the dissolution of the peace treaty between the Klingons and the Federation.
In the fifth season premiere "Apocalypse Rising", it was suspected by Starfleet that Gowron, the leader of the Klingon Empire, had been replaced by a Founder. Captain Sisko led an undercover mission to the Klingon military headquarters of Ty'Gokor in order to expose the Dominion imposter, but he, Worf, Odo and O'Brien were discovered and captured. "Martok" claimed that he believed that Gowron was indeed a changeling and released the team, leading them back into the Hall of Heroes so that they could kill the Chancellor. Worf challenged Gowron to honourable combat and Martok became confused as to why he didn't just shoot him. Odo mentioned that it was a matter of "Klingon honour", that's why Gowron didn't simply let his bodyguards kill the intruders. The Founders were unconcerned about such concepts and that was what gave the Martok changeling away. Odo called out to Worf that Martok was the changeling and the changeling responded by trying to strangle Odo (which could have worked since Odo had been turned human by the Founders at the time). Seeing Martok's arm turn into a long, gelatinous tentacle, several of the Klingons in the hall drew their disruptors and fired on the changeling. The changeling was bombarded with energy pulses and, after a few seconds and a dozen shots, exploded.
The real General Martok would later be found alive during the episode "In Purgatory's Shadow". He had been held prisoner in a Dominion internment camp for two years and forced to fight against Jem'Hadar soldiers in hand-to-hand tournaments. He had lost one of his eyes in one such fight. With the aid of Worf, Garak, and Dr. Bashir, Martok escaped from the prison camp and returned to the Klingon Empire, helping them to re-establish their alliance with the Federation and would go on to become a hero of the Dominion War in DS9's latter two seasons.
Torga IV changeling
In the fifth season episode "The Ship", a Jem'Hadar attack ship carrying a Founder crashes on the planet of Torga IV. The entire Jem'Hadar crew is killed but the changeling just barely survives, though it suffers significant injury. When Captain Sisko and his crew investigate the crashed ship, the Founder disguises itself as part of a bulkhead. A Jem'Hadar strike force later arrives lead by the Vorta Kilana, who tries to negotiate with Captain Sisko by allowing him to salvage the ship in exchange for letting her find an "item" aboard it. She does not mention a changeling is aboard since she fears that Sisko might take it hostage or kill it. It is not until after the death of Crewman Muniz that Sisko's officers discover the changeling, which begins to lose its form in front of them. The Founder dies and turns to ash as it does so. Kilana then beams into the ship and tells Sisko that her troops killed themselves because they allowed a Founder to die. Kilana is unable to stop Sisko from taking the ship now, but requests that she be allowed to take some of the Founder's remains, which Sisko allows. Kilana and Sisko then part ways and are left to contemplate on the fact that if the Starfleet and Dominion personnel had just trusted each other, no one would've had to die.
Centuries ago, the Founders sent out 100 infant changelings into the galaxy to learn about other races and gauge how threatening they could be. In the episode "The Begotten", one such infant changeling appears on Deep Space 9 having fallen into Quark's possession, who then sells it to Odo. The changeling is sick, having been exposed to high levels of tetryon particles. After the isotopes are purged from it, Odo - who had been turned human by the Founders months prior to this event - tried to communicate with the changeling and teach it how to shape-shift. He did this with the help of Dr. Mora, the Bajoran scientist who had discovered him years ago, and together they studied the changeling and helped it grow. Unfortunately, the infant grew sick again and its morphogenic matrix began to destabilise. Before it died, as Odo held the small blob of goo in his hands, the changeling integrated itself into Odo's body. Although the changeling perished, it restored Odo to his changeling state.s
Dr. Julian BashirIn the episode "In Purgatory's Shadow", Worf and Garak enter the Gamma Quadrant in response to a coded SOS from Enabran Tain, former head of the Obsidian Order who led the combined Cardassian-Romulan attack on the Founders' homeworld two years previously. During their search, they encounter a large Dominion fleet and are captured. They are taken to Internment Camp 371 where they find that they aren't the only people from Deep Space 9 being held prisoner: Dr. Bashir (portrayed by Alexander Siddig) is there too and has been there for over a month. This, of course, means that the Bashir back on the station is a changeling infiltrator.
The changeling impersonating Dr. Bashir is about to carry out his mission, which is to cripple the Federation and the Klingon Empire in one fell swoop by gathering their combined fleets in the Bajoran system (made possible by the Dominion's sudden incursion into the Alpha Quadrant) and destroying them all by causing the Bajoran sun to go supernova. The changeling hijacks the runabout Yukon and flies toward the sun, carrying a trilithium bomb aboard that will stop all fusion within the star, causing it to collapse. Before it can succeed, however, the real Dr. Bashir, Worf and Garak escape from the Dominion prison camp and warn DS9, prompting the Defiant to pursue the Yukon and destroy it, and the changeling with it.