Underworld ( TV story ) 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 . The story also advanced the mythos of the Time Lord s by explaining the origin of their non - interference policy . " Underworld " used by far the most colour separation overlay ( CSO ) in the programme ' 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 " . Synopsis . On the edge of the universe , the Doctor , Leela and K9 encounter a Minyan ship on an epic quest to find their race banks . . . but their people have encountered the Time Lords before . Plot . Part 1 . 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 . K9 informs him that there is a nearby spacecraft and they are both heading into a spiral nebula . The TARDIS escapes and materialises aboard the vessel , the " R1C " . When its crew hears the TARDIS , they recognise 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 civilisation develop before being rejected . This 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 bank s 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 around the ship , the Doctor suggests using the ship ' s weapons to blast itself free . Unfortunately , the laser cannon starts to burn through the hull as well . . . Part 2 . The laser punches through just in time . The " R1C " escapes , only to encounter another planet 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 Trog s 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 Seer s 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 . . . Part 3 . 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 dragon s which guard the Citadel , but it is merely a defence 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 . . . Part 4 . 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 recognises 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 grenade s 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 realises the real race banks are missing . When the grenades explode , they destroy the planet . The explosion is enough to propel the underpowered " 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 . Home video and audio releases . DVD releases . A North American / Region 1 release of the story occurred in July 2010 , but in that region the story was released on its own and not as part of a box set .