Li H'sen Chang with Mr. Sin

The Doctor and his travelling companion Leela arrive in Victorian London, at the end of the 19th Century, so that she can experience how her ancestors lived. After seeing and advertising poster The Doctor decides to take Leela to a music hall where a stage magician called Li H’sen Chang is performing.

But on their way to the Palace Theatre, The Doctor and Leela encounter a group of Chinese men who have apparently killed a cab driver. They attempt to silence The Doctor and Leela but are frightened away by the distant whistle of the approaching police. In the confusion Leela manages to incapacitate one of the thugs until the police arrive. When The Doctor and Leela arrive at the local police station they get to meet Li H’sen Chang, who has been called in to act as an interpreter, but unbeknownst to everyone he secretly gives the captive henchman a pill of concentrated scorpion venom which he takes immediately and dies. The Doctor, upon a brief examination of the body finds a scorpion tattoo - the symbol of the Tong of the Black Scorpion, devout followers of an ancient god Weng-Chiang.

When the body is taken to the local mortuary, along with the body of the cab driver, The Doctor decides to go there so as to investigate and it is there that he learns of other disappearances. He also makes friends with the police pathologist, Professor Litefoot who has discovered that hairs taken from the clothing of a dead body found floating in the Thames seem to have originated from a very large rat.

Leela, Professor Litefoot and The Doctor

The Doctor decides to explore the sewers and his investigations confirm that there are indeed giant rats down there. He also notes that the course of the river Fleet, which empties into the Thames, runs directly underneath the Palace Theatre which it seems is central to the numerous disappearances of young women in the area. The Doctor therefore decides to take Leela to the Palace Theatre so that they can investigate further.

What The Doctor does not realise at first is that the cab driver, who was killed by the Chinese men they encountered when they first arrived, had earlier visited the Palace Theatre to confront Li H’sen Chang about his wife’s disappearance as he was convinced he is involved with the string of missing women in the area. After threatening to report Li H’sen Chang, to the police if she was not returned to him, Li H’sen Chang, fearful of discovery, had sent his men to kill the cab driver.

Leela

The cab driver is correct and The Doctor and Leela soon discover that Li H’sen Chang is procuring the young women for his master, the ancient Chinese god Weng-Chiang who he believes has apparently been reincarnated on Earth. However, Weng-Chiang is in fact Magnus Greel, a war criminal from the 51st Century who had fled from the authorities in a time cabinet. However, the technology used in the time cabinet is based on ‘Zygma Energy’ which is unstable and has disrupted Magnus Greel’s molecular structure resulting in him needing to feed on the life-force of others - hence his use for the young women.

Magnus Greel has been searching for his precious time cabinet which, while he was in China and was taken from him by Chinese Imperial soldiers, and which in turn had been given by the Imperial Court to Professor Litefoot's parents as a gift. Using a locator he discovers that the time cabinet is in London and now in the possession of Professor Litefoot. So as to retrieve his time cabinet Magnus Greel infiltrates Professor Litefoot’s home with the help of Li H’sen Chang’s ventriloquist doll Mr. Sin. But Mr. Sin is actually an advanced computerised children’s plaything from the future with the cerebral cortex of a pig. The Doctor recognises it as being better known as the Peking Homunculus, a vile thing that almost caused World War Six when its organic pig part took over the toy’s functions. While in Professor Litefoot’s house Mr. Sin advances on Leela with a knife but is forced to make a hasty retreat when Professor Litefoot arrives.

Meanwhile The Doctor, aided by the Palace Theatre’s owner, Henry Gordon Jago, has located Magnus Greel’s hideout in the sewers underneath the theatre. However, they discover that Magnus Greel has already fled his lair, abandoning Li H’sen Chang who tries to escape into the sewers. But in doing so Li H’sen Chang is mauled by one of the giant rats - products of Magnus Greel’s experiments which he also used to guard his sewer hideout.

Jago and Litefoot

After locating the time cabinet Magnus Greel arranges for it to be stolen. When it is finally in his procession Magnus Greel prepares to travel back to his own time but discovers that the key to the time cabinet – a vital component which he needs to use the time cabinet - has been left behind in his hideout under the Palace Theatre. Unbeknown to Magnus Greel, while The Doctor and Leela try to find Magnus Greel’s new hideout, Henry Gordon Jago discovers the bag containing the key and he takes it to Professor Litefoot’s house. Realising its’ importance Professor Litefoot and Henry Gordon Jago return to the theatre and lay in wait for anyone who visits the theatre to retrieve it so they can follow him back to Magnus Greel’s new hideout. However, they are captured for their efforts.

Meanwhile, The Doctor and Leela happen upon Li H’sen Chang, in an opium den, who tells them that Magnus Greel can be found in the House of the Dragon but he dies before telling them its exact location. They then return to Professor Litefoot’s house where they find a note, left by Henry Gordon Jago, and the key to the time cabinet. They decide to wait for Magnus Greel and his henchmen. When they arrive, The Doctor uses the key, a fragile crystal known as a Trionic Lattice, as a bargaining chip. He asks to be taken to the House of the Dragon, offering the key in exchange for Professor Litefoot’s and Henry Gordon Jago’s release. Instead, Magnus Greel overpowers The Doctor and locks him in with the two amateur sleuths.

Leela, who had been left at Professor Litefoot’s house at The Doctor’s behest, decides to follow them but she is captured and finds herself placed in Magnus Greel’s life-essence extraction machine but before her life essence is drained, The Doctor, Professor Litefoot and Henry Gordon Jago manage to escape and rescue her. The Doctor then tries to convince Magnus Greel that using the time cabinet will create a catastrophic implosion. But Magnus Greel does not believe him. Mr. Sin (whose pig aspect has taken over making the computerised homonculus become unstable and dangerous) then uses a laser, that is part of a large statue of a dragon, to attack everyone. However, in the confusion, The Doctor defeats Magnus Greel by pushing him into the life-essence extraction machine, thus damaging it and causing it to overload. Having fallen victim to his own machine, Magnus Greel suffers cellular collapse and disintegrates in front of their astonished eyes. Mr. Sin then attacks The Doctor but he manages to disconnect its circuitry thus rendering it inanimate. The Doctor then brings the Zygma Experiment to a permanent end by destroying the Trionic Lattice.

Outside, as the London fog closes in, The Doctor and Leela, accompanied by Henry Gordon Jago and Professor Litefoot, take their leave and enter the TARDIS. As the TARDIS dematerialises Professor Litefoot is amazed, but Henry Gordon Jago takes it all in his stride and he comments that even Li H’sen Chang would have appreciated the stunt he has just witnessed.