Underworld was the fifth story of Season 15 of Doctor Who. It was the first over which script editor Anthony Read had complete creative control. Read asked writers Bob Baker and Dave Martin to make the script an explicit allegory of the myth of Jason and the Quest for the Golden Fleece.
Underworld used by far the most color separation overlay (CSO) in the program’s history. Rampant inflation had eaten away at the budget for the back half of the season and using CSO to create virtually every shot of the alien world featured in the story was the only feasible way to preserve the production of The Invasion of Time. The story also advanced the mythos of the Time Lords by explaining the origin of their non-interference policy.
Aboard the TARDIS, the Fourth Doctor is painting and Leela is playing with the console. When they reach the edge of the cosmos, the Doctor is in awe, but K9 informs him that there is a nearby spacecraft and they are both heading into a spiral nebula. The TARDIS escapes and materializes aboard the vessel, the R1C. When its crew hears the TARDIS, they recognize it as the technology of the gods.
The Doctor, now in the ship’s cargo hold, determines that it is from Minyos, a planet on the other side of the universe. The Time Lords once helped the Minyan civilization develop before being rejected, which led to their non-interference policy after the Minyans destroyed their own world. Leela uses a shield gun to blast the door open and they head for the bridge. As the crew try to avoid the nebula, the Doctor offers to help. Herrick holds a weapon on him, until Captain Jackson orders Orfe to use the pacifier to subdue Herrick and Leela. Jackson explains that the R1C has been searching for another ship, the P7E, for a hundred thousand years. While the crew can regenerate endlessly, their ship is failing. The P7E contains the genetic race banks needed to rebuild Minyos.
The Doctor connects K9 to the failing systems and the R1C escapes the nebula. However, the signal from the P7E is still coming from inside and Jackson orders the ship to go back in. With K9’s help, the ship survives the journey, but its huge mass starts to attract debris. As the rock builds up, the Doctor suggests using the ship’s weapons to blast itself free, but the laser cannon starts to burn through the hull as well…
The laser punches through just in time. The R1C escapes, only to encounter another planetoid that has built up over centuries. The Doctor suggests this could be the P7E. With no fuel left, the R1C penetrates the soft surface of the planet and heads for the core.
In tunnels under the planet’s surface, slave workers known as Trogs are buried by a rockfall. Guards are dispatched to restore order and accuse a Trog named Idmon of heresy for suggesting there is a sky. His son, Idas, runs away from the guards.
Jackson and his crew explore the tunnels and spot security cameras, suggesting a technologically advanced society. The Doctor and Leela see the guards chasing Idas and lead them away, losing them and finding Idas in the airlock of the R1C. The Doctor tells Idas about his people’s past and learns about the legends of the Sky Gods and the Seers who rule the society on behalf of the Oracle.
As Herrick scouts ahead, he is accosted by guards. They don’t believe his story and attack him. He easily deflects their fire, but the head guard, Rask, orders the tunnel sealed off and gas pumped in. The Doctor detects the gas. He finds the environmental controls and tries to reverse the flow, but cannot concentrate. He slumps over the machinery…
At the last second, the gas clears. Jackson, Orfe and Tala free Herrick; the guards are overcome by their own gas. When the Doctor wakes, Idas tells him about the punishment his father will endure for his heresy. Idmon will be sacrificed in the Citadel to appease the Oracle. A sword will be dropped on him. The Doctor sends K9 to find Jackson, while he takes Idas and Leela to stop the sacrifice.
Idas warns them of dragons which guard the Citadel, but it is merely a defense mechanism that Leela makes short work of with the shield gun. When they reach the planet’s core, which has zero gravity, they float down to the Citadel. At the sacrifice, the Oracle speaks to the Seers, who light a lamp to burn through the strap holding the sword. The Doctor’s party are taken to the sacrifice by the guards. The lamp is moved at the last moment. Idas incites a rebellion and frees his father. Jackson’s crew arrive to assist. Herrick remains behind as a rear guard to allow the rebels to escape, but he is captured.
The freed Trogs explain the endless futility of their lives, working for no purpose. Jackson agrees to help the Doctor infiltrate the Citadel and find the Oracle. The Doctor and Leela lie in an ore cart headed for the Citadel.
The Seers torture Herrick. They refuse to believe his story of a millennia-long quest. They remove their ceremonial masks to reveal strange, metallic faces.
As the ore cart approaches the Citadel, the automated system tips the contents of each cart into a crusher, including the Doctor and Leela…
The Doctor and Leela hang onto a ledge by their fingertips and are rescued by the R1C crew. Jackson agrees to hold off the guards while the Doctor’s party head for the Oracle.
In the Citadel, the Seers suggest letting Herrick have the race bank cylinders, since they are of little value and not worth the risk. The Oracle releases the cylinders and offers to let Jackson have them if he will leave. Jackson agrees and takes the cylinders back to his ship. The Doctor speaks to the Oracle. He recognizes it as a deranged computer programmed to protect the race banks at all costs; therefore the cylinders given to Jackson must have been fakes. He takes the real ones.
Jackson prepares his ship for launch, but stops when K9 identifies his two cylinders as fission grenades capable of destroying a small planet. The Doctor takes the grenades back into the tunnels, where he encounters Rask, sent to retrieve the race banks. The Doctor hands over the grenades instead and they are returned to the Oracle chamber. The Doctor, Leela and Idas help the Trogs escape the tunnels and convince Jackson to take them with him, since they are the descendants of the P7E crew.
The R1C takes off before the Oracle realizes the real race banks are missing. When the grenades explode, they destroy the planet. The explosion is enough to propel the under-powered R1C clear of the nebula. The ship heads for Minyos II, a mere 370 light years away. As they leave, the Doctor calls the captain “Jason”. He has to explain to Leela about the legend of the Argonauts. He wonders if it is also a premonition.