In the far future, the citizens of Pluto are taxed heavily, including Cordo (Roy MacReady), who is so overwhelmed he decides to take his own life. He is interrupted by the Doctor and Leela, the TARDIS having landed on a high rooftop. They save him and learn that six artificial suns have been built around Pluto to bring life to that planet, but the owners of the suns are bleeding the people dry. Each megalopolis is ruled by a tax Gatherer and all report to a Collector. Some people live in dark tunnels beneath the city and never see sunlight. The Doctor, Leela, and Cordo meet the renegades there, thieves led by the brutal Mandrel (William Simons). The Doctor must pay him or Leela will be killed. The Gatherer of Megalopolis One, named Hade (Richard Leech) has noted the landing of the TARDIS and is tracking K9, who has gone in search of the Doctor. K9 unwittingly leads the Gatherer, who suspects that arms-dealers are at work, to the Doctor and Cordo at a cashpoint. As the Doctor attempts to use false credentials to get cash for Mandrel, he is trapped in a booth and gassed. He falls unconscious.
When the Doctor wakes, he is in a correction centre with another prisoner, Bisham (David Rowlands). They are facing torture, but the Doctor is worried about Leela. However, Leela has defended herself nicely and is beginning to be respected by the criminals. Cordo returns to the undercity with news that the Doctor is captured. Leela sets out to save him, but only Cordo is brave enough to join her and K9. The doctor is released after questioning, but the Gatherer is watching him, thinking he will lead the authorities to a conspiracy against the Company. Leela, Cordo, and K9 attack the correction centre, but the Doctor is gone. They free Bisham, and they are trapped by Inner Retinue troopers. Leela leads an attack on the guards but is injured and falls from a vehicle they have boosted. The Doctor returns to the undercity. Mandrel refuses to believe he has been released and threatens him. Cordo uses a stolen blaster to force Mandrel to release him. He persuades the undercity dwellers to revolt, attacking the main control area where Company engineers produce a drug that keeps the people in fear. Leela is presented to the Collector, an odious little creature in a power-chair. He orders Leela killed. The Doctor frees her, but the microphones supposed to record her death-screams pick up Mandrel speaking to the Doctor. The Collector is enraged, but the revolution has begun. Hade is killed—thrown from the roof by the mob--and his underling Marn (Jonina Scott) joins the revolution.
Leela and the Doctor penetrate the Collector’s palace and the Doctor sabotages the computer system. The Collector arrives and the Doctor learns he is a seaweed-like poisonous fungus from the planet Usorius. Before he can gas the population, Cordo and the rebels help the Doctor defeat the Inner Retinue army. The collector, shocked to learn he has lost all his money, shrinks down into his poisonous seaweed size, and is sealed up. The Doctor, Leela, and K9 depart in the TARDIS.
The numbers of corridors in the city are those of tax-forms in the U.K. People running about are thus lost in a maze of government forms. In fact, the entire story was created because writer Robert Holmes was angry about his taxes. Thus, the Inner Retinue, named after Inland Revenue, and the resemblance of Collector, with his bushy eyebrows, to Chancellor of the Exchequer at the time, Denis Healey. This was Louise Jameson’s favorite story, probably because she got to run around fighting and shooting guns well out of the larger-than-life shadow of Tom Baker.