'Mawdryn Undead' (4 Parts) Story Code '6F' by Peter Grimwade

In order to avoid a collision with a huge spaceship, the TARDIS materialises inside it. The Doctor discovers that the ship is bound on an eternal fixed warp ellipse orbit around Earth, and that a signal from the planet is preventing the TARDIS from leaving. He uses the ship's transmat capsule to travel to the surface in order to deactivate the signal, and arrives near a public school in 1983, where he meets the Brigadier, who has retired from UNIT and is now the school's maths teacher, and Turlough, a young student. Unknown to the Doctor, Turlough is in fact an alien who has been exiled to Earth; he has been contacted by the evil Black Guardian, who has struck a deal with him - in return for helping the youth to escape from his imprisonment, Turlough must kill the Doctor. The Doctor deactivates the signal, and Tegan and Nyssa take the TARDIS to join him, but instead the warp ellipse causes them to arrive in the year 1977, where they meet that era's Brigadier. Returning to the TARDIS, they find a burnt and blackened figure that tells them he is the Doctor - he has been forced to regenerate following a transmat accident, but the process has gone horribly wrong. While they use the TARDIS to take him and the Brigadier back to the ship, the Doctor, Turlough and the 1983 Brigadier also return using the transmat. Here they discover that the 'Doctor' is in fact Mawdryn, a renegade scientist who, together with eight of his fellows, stole a Metamorphic Symbiosis Regenerator from the Time Lords in order to attempt to make themselves live forever. However, their plans went awry, and they are now forced to live out the rest of their lives in an agonising state of perpetual regeneration. Mawdryn and his fellow mutants want the Doctor to give them his life force, hoping that his remaining regenerations will enable them to bring an end to their torment. The Doctor refuses, but when he realises that Tegan and Nyssa have also been contaminated by the mutation, and will die if they leave in the TARDIS, the Doctor, he reluctantly aggress. However, the two different Brigadiers meet, and the energy discharge unleashed in the resulting Blinovitch Limitation Effect is enough to kill the aliens, cure Tegan and Nyssa and release the Doctor from his obligations. The Doctor returns the Brigadiers to their correct times, and then, unaware of his murderous intent, allows Turlough to join the TARDIS crew.

Peter Davison (The Doctor), Janet Fielding (Tegan), Sarah Sutton (Nyssa), Mark Strickson (Turlough), Nicholas Courtney (Brigadier Lethbridge-Stewart), Valentine Dyall (Black Guardian), Angus MacKay (Headmaster), Stephen Garlick (Ibbotson), Roger Hammond (Doctor Runciman), Sheila Gill (Matron), David Collings (Mawdryn), Peter Walmsley (1st Mutant), Brian Darnley (2nd Mutant)

Produced by John Nathan-Turner
Directed by Peter Moffatt

TX:
Part 1 - 1st February, 1983 @ 6.50pm - 7.15pm
Part 2 - 2nd February, 1983 @ 6.45pm - 7.10pm
Part 3 - 8th February, 1983 @ 6.50pm - 7.15pm
Part 4 - 9th February, 1983 @ 6.45pm - 7.10pm

Notes:
*Featuring the Fifth Doctor, Nyssa, Tegan, the Brigadier and introducing Vislor Turlough

*Part 1 of the 'Black Guardian Trilogy'