The White Guardian (Cyril Luckham) recruits the Doctor to find the six disguised segments of the Key to Time. He is provided with an assistant, Time-Lady Romanadvoratrelundar (Mary Tamm), whom he calls Romana. She is the third Time-Lady to appear in the entire series, after Susan, the First Doctor’s granddaughter, and Lady Rodan in the Invasion of Time. The White Guardian provides a kind of wand which will locate the Key to Time sections and remove their disguise, and he warns that balance in the universe requires a Black Guardian as well, who is on the same quest for nefarious purposes. The device, when planted in the TARDIS console, indicates one segment on the planet Ribos.
Ribos is an ice-covered planet with a medieval culture that is unaware of space-travel. An Earthman named Garron (Iain Cuthbertson) is trying to sell the entire planet (which of course he does not own) to an unsuspecting tyrant called the Graff Vynda-K (Paul Seed), because it contains a rare mineral called jethrik, invaluable for faster-than-light travel. Graff Vynda-K sees a quantity of jethrik among the Ribos crown jewels but does not know it was planted there by Garron’s assistant Unstoffe (Nigel Plaskitt). The locator points to this chunk of jethrik as the disguised Key to Time.
The Graff Vynda-K has put a large deposit on the planet Ribos, which is stored in the room with the crown jewels, watched by Ribos guards in the daytime and a rubber carnivorous beast called a shrivenzale at night. Unstoffe distracts the beast, takes the piece of jethrik and all the money. The Graff discovers Garron’s listening device in his room and has him arrested him along with his supposed accomplices, the Doctor and Romana. Did nobody warn Romana that the Doctor gets arrested everywhere he goes? Unstoffe hides out with Binro (Timothy Bateson), an outcast and heretic who believes that Ribos is a planet orbiting a star. In a rather touching bit of dialogue, Unstoffe confirms this because he comes from Earth. The Ribos guards summon a Seeker (Ann Tirard) who locates the fugitives. Using the listening device, Garron warns them and Binro leads Unstoffe into the catacombs beneath the city.
The guards fear this place because of the ghosts, and because the Seeker warns them that all who enter but one will die. So, the Graff kills her. K9 helps the Doctor, Romana, and Garron escape prison and enter the catacombs. The guards destroy the entrance and the ceiling collapses, trapping everyone inside. The Graff has the money and the jethrik. Before escaping, he gives a bomb to his last guard to kill himself with, just to cover his beloved master’s escape. Nice Guy! But the guard turns out to be the Doctor in disguise, who, with sleight-of-hand, swaps the bomb for the jethrik. Graff goes off down the tunnel, congratulating himself, and is blown to bits. Garron and Unstoffe steal the Graff’s ship and head off on a new scam, while the Doctor and Romana transform the jethrik into the first piece of the Key to Time.
This was the first installment of a season-long story, something rarely attempted. Later, the Sixth Doctor (Colin Baker) starred in The Trial of a Time Lord. In the Doctor Who tradition of well-drawn and acted minor characters, we have Garron and Unstoffe as loveable rogues, Binro as the wistful heretic, the crazy Seeker, and the Graff Vynda-K as the over-the-top warlord. The Doctor now sports an extra-long scarf. Mary Tamm was generally liked as Romana. She wore gorgeous and elegant costumes, had an aristocratic air about her, and was not the slightest bit intimidated by the Doctor. When he claimed to be 756 years old, she pointed out that he was 759 and beginning to lose his memory. She was first in her class in Time-Lord Academy, and he barely squeaked by. She was created because Elizabeth Sladen declined to return as Sarah Jane Smith.