The Master aided Grinmal, creating a wormhole between Oseidon and Derbyshire. He then double-crossed the Master and imprisoned him with Leela. He allied with the Master's former ally Colonel Spindleton, promising him control of Great Britain after the invasion.
However, the Master who had been imprisoned was an android. He and Leela escaped, and reprogrammed the other androids to destroy the Kraal army. But when he tried to deactivate the androids afterward, he turned himself off as well.
Grinmal, now the only survivor, allied himself with Leela and the Fourth Doctor, so he could stop the Master's real plan, which involved destroying Oseidon. While trying to stop the plan, Grinmal was shot by Spindleton, but the Master's plan was foiled. (AUDIO: The Oseidon Adventure)