These three stories saw the return of the Doctor's arch-enemy, The Master, as well as the transition from Tom Baker's Doctor to Peter Davison's. The Keeper of Traken: A distress call brings the Doctor (Tom Baker) to the tranquil planet of Traken, where a living statue poses a deadly threat. (4 eps, 98 mins) Logopolis: The Doctor's (Tom Baker) plan to enlist the help of Logopolis's mathematicians for a small favor become sidetracked when the Master's interference leads to disaster on a universal scale. (4 eps, 98 mins) Castrovalva: The Doctor's (Peter Davison) regeneration is failing, and his last hope rests with Nyssa and Tegan, who struggle to steer the TARDIS to the remote haven of Castrovalva. (4 eps, 96 mins)