The Tardis is flying through space when suddenly Nyssa notices a ship is flying directly towards them. Though the probability is high, the Doctor takes evasive maneuvers to avert a collision and materializes the Tardis inside the ship. Stepping out of the Tardis, Tegan thinks the ship looks more like the Queen Mary, for how extravagant it is. As they continue to explore, they find no one. That's when Tegan corrects herself and now thinks the ship reminds her more of the Mary Celeste. When they try to take off again, the Tardis is stuck. There is some interference holding the Tardis in place.
Meanwhile, on Earth. Turlough is a bored school student who coaxes his friend "Hippo" into taking a vintage car for a joy-ride. Hippo is horrified when Turlough does not return immediately s he suggested he would. Turlough's cockiness soon fades as he suddenly has to veer off the road to avoid a collision. In an instant, he's hovering over his body while a sinister figure strikes up a bargain with him. Turlough's 'Black Guardian' wants him to kill the Doctor, in exchange for giving him free passage back to his home planet. Turns out, Turlough is a marooned alien and very bored with Earth. Reluctantly, Turlough agrees when he hears how evil the Doctor is.
Back on the ship, the Doctor finds the source of the interference, but he must use the ships transport pod to travel to Earth and disable it, so the Tardis can follow him. He instructs Tegan and Nyssa to stay in the Tardis and once released, it will go to Earth where they will meet up with the Doctor. The girls do as instructed but when they are on Earth and find the transport pod, they find a badly burnt figure laying on the floor. The girls immediately think something happened to the Doctor, causing him to regenerate. However, it's not long before Tegan begins to have her doubts about who this character is.
Once on Earth himself, the Doctor meets Turlough while he's trying to disable the interference. Turlough is about to smash the Doctor's head in with a rock when an explosion takes place knocking both of them back, and the rock out of Turlough's hands. However the Tardis should be free, so when it doesn't show up the Doctor is worried. Suddenly, the owner of the vehicle Turlough wrecked shows up looking for Turlough, it's none other than Brigadier Lethbridge-Stewart. The Doctor is happy to see his friend again, but the Brigadier has no idea who the Doctor is.
Fearing for the Doctors life, Tegan runs off looking for help. It's not long before she runs into a teacher who is willing to help her. Too her surprise, the teacher knows the Doctor. It's Brigadier Lethbridge-Stewart who's just retired from UNIT. He's anxious to meet the Doctor. But when they arrive at the Tardis, the person they found has continued to regenerate and despite his head, looks much better. He pilots the Tardis back to his ship, bringing along the Brigadier. However this Brigadier is from 1977, which is dangerous because the Doctor has invited the Brigadier he met from 1983 to travel back with him to the ship. Turlough accompanies the Doctor and Brigadier.
Back on the ship, the Doctor is reunited with Tegan and Nyssa and horrified to realize they each brought a Brigadier with them. To make matters worse, each has wandered off. The Doctor sends Turlough off to help keep the two Brigadier's separate. As Turlough does that, the Doctor discovers the identity of the person Nyssa thought was the Doctor. His name is Mawdryn, along with his fellow shipmates (who were hiding earlier) stole Gallifreyan technology in the hopes of becoming Timelords. The result worked to some degree, they are now cursed with perpetual regeneration for all eternity. The prospect is too much for them to bear and they wish to be released from their curse. The ask the Doctor for help.
The Doctor refuses to help Mawdryn and the rest of his crew, because to do so will require him to be hooked up to the machine and burn off the remainder of his regenerations to cure them. Mawdryn says the Doctor can only help of his own free will, so they allow him to leave. But to the Doctors horror, he finds Tegan and Nyssa have been infected by Mawdryn and if they travel in the Tardis they will either age or deage depending on which direction the Tardis travels. The only way to cure both of them is to hook them up to Mawdryn's machine also. Reluctantly the Doctor returns to sacrifice himself and cure everyone else.
With the two Brigadiers still wandering around, Turlough is doing what he can keep them separate. While doing so, he has another run in with the Black Guardian who is dissatisfied with Turlough. He once again warns Turlough of the bargain they struck, but Turlough thinks its enough to he has marooned the Doctor. He's increasingly finding it more and more difficult to want to kill the Doctor. When he tells the Black Guardian the Doctor doesn't seem evil, the Black Guardian tells him the Doctors good is his evil.
Turlough succeeds in trapping the younger Brigadier in a room as the older Brigadier now helps the Doctor set up the Mawdryn's equipment. But the Brigadier is resourceful and soon finds a way out of the room he's been trapped in. He finds the Doctor and his older self. Ignoring the Doctor's pleas to leave, he's bewildered by his older self and reaches out to touch himself. There is an explosion which takes place at the exact moment the Doctor was offering up his future regenerations. The explosion is transferred into the equipment and thus saves the Doctors future regenerations. As Mawdryn dies, he thanks the Doctor. Returning each Brigadier to their respective time zone, Turlough asks the Doctor if he can accompany him. The Doctor agrees, to the dismay of Tegan who doesn't trust him.
WHO: Vislor Turlough
PLANET OF ORIGIN: Trion, circa 1983
REASON FOR JOINING: Turlough is an alien exiled to Earth, but hates his boring life. When the Black Guardian offers him a chance to leave the planet, he hastily agrees and thus becomes involved with the Doctor. Having agreed to kill the Doctor, Turlough asks to join the Tardis crew in hopes of fulfilling his end of the agreement.