Luco was the trusted assistant to Sardon, the Time Lord responsible for recruiting the Second Doctor and delaying his execution. As he was on assignment by Sardon to observe suspicious temporal interference, he met with one of the Players responsible for the interference, the Countess. She seduced him into being her ally.
Luco betrayed Gallifrey with his actions many times, hoping to eventually replace a disgraced Sardon from the outcome. First he simply failed to tell the truth when he witnessed the interference of time by the Players. Then he supplied the Countess with vital information about the Second Doctor's plans, which came close to killing him several times. He helped the Countess by aggressively plotting against the Doctor's life, calculating that a renegade would get little attention. He used a Time Scoop as a method of murder, setting a vampire and a Raston Warrior Robot after the Doctor but each time the Doctor defeated their plans.
The Doctor demanded that Luco share in the responsibility for Serena's murder by the Players, and exposed his duplicity. Sardon did not hesitate to place him on trial for high treason and ordered the Mind Probe to be placed on him during his inquiry.