In 1983, Vislor Turlough (Mark Strikson) a stranded alien posing as a human student, is offered passage off the Earth by the Black Guardian (Valentine Dyall), but he must kill the Doctor. Mean while, the TARDIS is caught in the warp ellipse of a starliner trapped in time. Turlough arrives by Transmat, pretending to be lost. The Doctor instructs Nyssa and Tegan to stay aboard the TARDIS while he returns with Turlough to 1983 to fix the transmat point, hoping this will help the TARDIS to escape its entrapment. Instead, it materializes in 1977 at Turlough’s school. Searching for it, the Doctor visits his old friend Brigadier Lethbridge-Stewart (Nicholas Courtney), teaching at that school in 1983. The Brig seems to have lost his recent memory, but as the Doctor speaks of 1977, they begin to come back.
In 1977, Nyssa and Tegan leave the TARDIS and find a terribly disfigured man in the transmat capsule, whom they think is the Doctor. They seek help from the younger Brigadier and the fake Doctor (David Collins) urges them to return to the starliner, which they do, the Brig accompanying them. In 1983, the Doctor detects the TARDIS’ motion and he, with Turlough and the 1983 Brigadier, returns to the starliner in the transmat. The Doctor meets the other time-travellers and realizes there are two Brigadiers in the same space-time. He tries to keep them separated. Should they touch, the Blinovitch Limitation Field could cause catastrophic energy discharges. Who knew?
The figure pretending to be the Doctor reveals himself as Mawdryn, one of a number of scientists aboard the liner who were seeking the secret of regeneration. This went sideways and they are now in this deplorable state. The Doctor would like to help them without giving up his own future regenerations. Then Nyssa and Tegan suffer the same affliction, aging in reverse until they are children. Elsewhere on the ship, the two Brigadiers find each other. The flash of energy as they touch occurs exactly as the Doctor gives up his regenerations, which kills the suffering Mawdryn and his fellows, restores Nyssa and Tegan, and saves the Doctor. The younger Brigadier passes out, losing some of his memory. The TARDIS crew returns the Brigadiers to their respective decades and the Doctor accepts Turlow aboard the TARDIS, unaware of his deal with the Black Guardian.
This is the first of three stories called the Black Guardian Trilogy: Mawdryn Undead, Terminus, and Enlightenment. We are back to three companions now: the frustrated and shrill Tegan, the goody-two-shoes but resentful Nyssa, and the sullen Adric replaced by a companion actually out to kill the Doctor. Originally, William Russell as the First Doctor’s companion Ian Chesterton was to appear, but he was unavailable. He was a teacher, thus the school location. Then Ian Marter as Harry Sullivan, the Fourth Doctor’s early companion was considered, but it was finally decided to bring back Lethbridge-Stewart. Fans rejoiced. The story was complex but interesting and engaging.
Mark Strickson was blond, but his hair was dyed red to be different from Peter Davison. In his flashbacks, the Brigadier sees the four previous doctors. The scientists are in their state because Mawdryn stole a Metamorphic Symbiosis Regenerator and was unable to make it work properly. The Brigadier’s car is a blue 1929 Humber 16/50 open tourer, Imperial model. Apparently, he caught the antique car bug from the Third Doctor. In this, the 20th Season of Doctor Who, every story included something from the Doctor’s past, climaxing with the Five Doctors story.