When the Doctor decides to stop a war, the last thing he expects is to be trapped aboard a doomed spaceship with his oldest friend — who just happens to also be his oldest enemy. The Master is just as surprised: this isn't part of his master plan.
Liv tells Sally that the Master doesn't really need her but just someone to hold his test tubes and tell him how wonderful he is. This closely resembles what the Brigadier once told the Doctor about his companions. (TV: Terror of the Autons)