The Cortez Project was a conspiracy within UNIT during the 1990s. One of its superiors was Marianne Kyle.
They believed that aliens, even if they didn't plan on invading Earth, would usurp human culture and therefore should be killed.
They were encountered by the Seventh Doctor and Sarah Jane Smith in 1997. They wanted to destroy a Tzun vessel who were leaving Earth to return home. The Doctor stopped their plan to use the USS Westmoreland to destroy the ship before it left the atmosphere. Colonel Tsang, the officer in charge of the mission, said that the Cortez Project couldn't be stopped, and that they were everywhere, and then blew the side of her head off with a pistol.
The Doctor trusted Alistair Gordon Lethbridge-Stewart to dig up the other members of the Project. (PROSE: Bullet Time)