The glory days of the Skonnan Empire are long since past, but many of its citizens and soldiers yearn for those days of control and conquest. The arrival of the mysterious horned Nimon to Skonnos has, however, brought hope of imperial restoration. The fearsome creature from within its labyrinth has promised to rebuild the Empire providing it receives a series of tributes from the Skonnans and their fawning, arrogant leader Soldeed. This tribute is to consist of groups of youthful sacrifices from the nearby planet Aneth, as well as a supply of hymetusite crystals with each group. Young people have thus been abducted from there and transferred to the labyrinth of the Nimon. On the final collection, however, the interstellar craft bearing the sacrifices breaks down in space. The ancient war craft has simply worn out, and when the co-pilot over extends the engines, it becomes dangerous and kills the Pilot when one of its control panels explodes.
The Doctor, Romana and K-9 are in the TARDIS console room where he is making modifications to the ship. Various controls are disconnected and unfortunately the area of space he has chosen to materialise the ship in is perilously close to what seems to be the basis of a manufactured Black Hole. They are in danger of being drawn in. He extends the TARDIS door force field to a nearby spaceship – the Skonnan battle cruiser – and he and Romana board the aging warship. Once aboard the Doctor notices an abundance of radioactive hymetusite crystals and then soon finds a hold full of young prisoners. They are from the peaceful world of Aneth and one of them, Teka, has a seemingly misplaced faith in another prisoner, Seth, to try and free them from their incarceration. The Co-Pilot investigates the hold and there finds the Doctor and Romana among the “weakling scum”. He takes them at gunpoint to the bridge and forces them to help fix the stranded Skonnan ship. Romana offers to repair the ship using a hymetusite crystal and one is brought to her while the Doctor is permitted to return to K-9 and the TARDIS and assist from there. Aboard the TARDIS the Doctor is informed by K-9 about the collapse of the Skonnan Empire in civil war.
With the Skonnan craft repaired the Co-Pilot starts to move his ship away, stranding the TARDIS in space. With the TARDIS still not fully repaired and the gravity increasing, the Doctor and K-9 face obliteration as a vast planetary body advances directly toward them.
Using a cricket ball strategy, the Doctor bounces the TARDIS off the approaching asteroid. He then starts work on repairing the console in order to pilot the ship to Skonnos and confront the evil there. The planet is being dominated by the Nimon, which exists within its labyrinthine Power Complex into which only Soldeed is permitted to venture. The Nimon is angry when Soldeed reports the Skonnan craft with the last batch of sacrifices has not reached Skonnos and says it will withhold the arms that will help rebuild the Skonnan Empire until the ship arrives.
Fortunately for Soldeed, once he emerges from the Power Complex he hears from his guard captain, Sorak, that the ship has been found. The warship soon arrives on Skonnos and Soldeed leads the party of greeting, being unnerved to see Romana aboard. The Co-Pilot then lies that she is the cause of all the problems on the ship, being a pirate who stole aboard and killed the Captain. Soldeed does not believe this and then forces the Co-Pilot into the Nimon Power Complex where he is sure to be killed. Moments later Romana and the Anethans are loaded up with hymetusite and also sent into the maze. Shortly thereafter the TARDIS materializes in the central square of Skonnos and the Doctor emerges. He is taken to Soldeed but soon escapes and heads into the Power Complex to escape his pursuers.
Deep in the Complex – whose walls seem to shift and change creating various patterns of progress that all lead to the Nimon – Romana finds the husks of previous Anethans, the life drained from them. The Co-Pilot also arrives, still pleading for his life, and when the Nimon appears too it despatches the desperate soldier first (as shown in the picture above) before turning its mighty horns on Romana and the cowering Anethans.
The Doctor arrives in the nick of time to distract the Nimon and thus save Romana, Seth and Teka who make a break for it after him, though the other Anethans are too scared to leave. He leads them deeper into the Complex and finds a power source close to the heart of the maze, but he needs a computer to interpret the machine and so blows his dog whistle to summon K-9. When the robot dog emerges from the TARDIS it encounters Soldeed, who immobilises it and takes it away for examination.
Back in the heart of the Complex the Nimon has now reached the power source room and starts manipulating the controls of the machine, which begins to cause the Complex to glow with energy. It also enables a shimmering tunnel to appear, down which comes a travel globe that bears two more Nimon. They announce to the other Nimon that the planet Crinoth is dying and that all the Nimon must continue the Great Journey of Life to Skonnos. Once the Nimon stalk away the Doctor examines the globes and pronounces that they are travelling vessels that have journeyed down a tunnel set between two black holes. By mishap the Doctor sends the globe down the tunnel with Romana in it but before he can reverse this Soldeed arrives and uses his staff to destroy the control panel.
Romana has arrived in the dying world of Crinoth where she encounters many Nimon who live as per their operation on Skonnos. Their equivalent of Soldeed is a broken old man named Sezom, who helped the Nimon establish themselves on his world and now knows they have destroyed it. He compares the Nimon to intergalactic locusts, swarming between planets and draining them of energy. He has also discovered that when jacenite is integrated into the staff that he was supplied with by the Nimon, it has the ability to stun them. He gives Romana an extra piece that he has, but is shortly afterward killed by a Nimon while helping her to escape.
Seth shoots Soldeed unconscious, and the Doctor then attempts to repair the transportation system. Just as he is about to complete the repairs, the Nimons return to the power source room and restrain him. However, they finish his work by reversing the tunnel, which brings Romana back in the capsule she had been waiting in on Crinoth. Romana tosses the jacenite to Seth, who now has possession of Soldeed's staff, and he uses it to stun two of the Nimons. Having managed to free himself from Soldeed's laboratory, K-9 arrives just in time to deal with the remaining one. Soldeed, having escaped from the power source room, has seen the multiple Nimons and his faith is badly damaged. He is shot down by Seth but in his death throes manages to trigger a chain reaction which will destroy the Complex. The Doctor and his party make their way out, using K-9 to work out a proper exit path through the labyrinth. They all escape and join up with the remaining members of the Skonnan military council, all of whom evacuate the main square as the Nimon Power Complex explodes.
Later in the TARDIS the Doctor reflects on their adventure and watch as Seth and Teka pilot a spacecraft away from Skonnos, having been granted their freedom. Elsewhere Crinoth can be seen disintegrating. It seems that the Nimon threat is over.
Doctor Tom Baker (Fourth Doctor)
Companions David Brierley (K-9 Mk. II)
Lalla Ward (Romana II)
Graham Crowden — Soldeed
Michael Osborne — Sorak
Malcolm Terris — Co-Pilot
Bob Hornery — Pilot
Simon Gipps-Kent — Seth
Janet Ellis — Teka
John Bailey — Sezom
Robin Sherringham, Bob Appleby, Trevor St John Hacker — Nimons
Clifford Norgate — Voice of the Nimon
Writer Anthony Read
Director Kenny McBain
Script editor Douglas Adams
Producer Graham Williams
Executive producer(s) None
Production code 5L
Series Season 17
Length 4 episodes, 25 minutes each
Originally broadcast December 22, 1979–January 12, 1980