The Son'a are the secondary antagonists in the movie Star Trek: Insurrection, a newly-introduced hostile species.
The Son'a were an offshoot of the Ba'ku people, who had objected to their leaders' refusal to use technology. They had attempted to stage a coup but had been defeated and sent into exile. They formed their own culture, becoming technologically advanced, until it reached the point where no-one, including the Ba'ku, was aware of their true origin. Deprived of the metaphasic radiation that had kept them young, they relied on medical techniques to artificially extend their lives, becoming more and more unrecognisable. Their culture was notoriously brutal, with them conquering two other races, the Tarlac and Ellora, and incorporating them into their own culture as a servant class.
Ultimately, the Son'a reached the conclusion that they needed to return to the Ba'ku planet in the Briar Patch, but discovered it would take ten years of normal exposure for the radiation to have any effect. They thus designed a metaphasic collector which would harvest the radiation but render the planet uninhabitable. Under the leadership of Ahdar Ru'afo, the Son'a formed an alliance with the Federation, represented by corrupt admiral Matthew Dougherty, to use a cultural observation as cover to surreptitiously remove the Ba'ku from the planet using a holoship projecting an image of their village and then activate the collector.
Their plan began to go wrong when the Enterprise crew discovered the plot and opposed it. Picard sent the Enterprise under Riker's command out of the Briar Patch to contact the Federation and plead the Ba'ku's case to them, while he and a small group of senior officers remained on the planet to delay the removal of the Ba'ku. With Dougherty's permission, Rua'fo sent two ships to forcibly escort the Enterprise back to the planet, but a battle left one destroyed and the other disabled.
The Son'a sent down troops and automated drones to implant the Ba'ku with trackers in order to beam them off the planet. The attempt only resulted in the capture of a small number, plus Picard. The Enterprise crew had scanned a fallen Son'a soldier and learned the truth of their origin, that they were the same species as the Ba'ku. With this information, and the realisation Rua'fo now intended to kill the Ba'ku, Dougherty tried to pull out of the deal but Ru'afo killed him.
Ru'afo gave the order to activate the metaphasic collector and kill all the Ba'ku remaining on the planet, but Picard, with help from Data, Worf and Ru'afo's second-in-command, Gallatin, managed to beam Ru'afo and his bridge crew to the holoship. Ru'afo beamed over to the collector to activate it manually and the Son'a managed to retake their bridge, but the ship was disabled when the Enterprise rammed it and Ru'afo was killed when the collector self-destructed. The remaining Son'a surrendered.
The Son'a were not seen again but were mentioned as allies of the Dominion, manufacturing ketracel-white for them.