Page "Mawdryn Undead" Paragraph 4

Ultimately, the Doctor learns the truth of the liner's crew, that they are several scientists that attempted to discover the Time Lord secret of regeneration, but ended up as disfigured creatures, unable to die.

The alien that Tegan and Nyssa met reveals himself as Mawdryn, and tries to convince the Doctor to give up his remaining regenerations to kill Mawdryn and his fellow sufferers.

The Doctor tries to leave with his companions in the TARDIS, but finds that Tegan and Nyssa are now suffering the same condition as Mawdryn, and de-age rapidly while the TARDIS is in the Time Vortex.

The Doctor realises he has no choice, and prepares to give up his remaining regenerations to not only kill the scientists but to also restore Tegan and Nyssa to normal.