In 1983, Vislor Turlough, a stranded alien posing as a human student, is given an offer by the Black Guardian for passage off Earth if he should kill the Fifth Doctor.
In 1977, Nyssa and Tegan leave the TARDIS and find a horribly disfigured man in the transmat capsule, who claims to be the Doctor in the midst of a regeneration.
The Doctor regroups with his companions; realising two versions of the Brigadier are aboard, he instructs them all to keep the two separated, as, should they touch, it could release a potentially catastrophic energy discharge due to the Blinovitch limitation effect.
The figure posing as the Doctor is forced to reveal himself as Mawdryn, one of several scientists aboard the liner who were trying to discover the Time Lord secret of regeneration.
Their experiments failed, and he and his fellow scientists have become immortal in this painful state and seek to die, but the Doctor determines the only way to do so is to give up his remaining regenerations.
He attempts to leave with his companions, but finds that Nyssa and Tegan suffer the same affliction as Mawdryn, ageing and de-ageing rapidly once in the Time Vortex, and quickly returns to the ship.
Some consideration was given to using instead the character of Harry Sullivan, who was a regular in the programme for a season in the mid-1970s, before the return of Lethbridge-Stewart was eventually decided upon.
It was released on DVD as part of the Black Guardian Trilogy on 10 August 2009 (Region 2),[4] with a commentary by Peter Davison, Mark Strickson, Nicholas Courtney and Eric Saward and an option to view the story with new CGI effects.