Mawdryn presents the Doctor with an ultimatum; give up his remaining regenerations or leave the ship and suffer the consequences. Meanwhile Turlough's stupidity puts the two Brigadiers at risk of meeting, angering the Black Guardian.
Mawdryn and his fellow immortals - stuck in ceaseless cycles of mutation and deterioration - are quite determined on the Doctor sacrificing his future for their sakes, giving him the choice between helping them or stranding Teagan and Nyssa with them on their infinite journey. The Doctor, however, seeks a third option.—statmanjeff