The hunt for the fourth segment of the Key to Time brings the TARDIS crew to 1930s Shanghai: the Tongs pursue their own agenda in the city's illegal clubs, while the Japanese Empire looms in the background.

While stealing a priceless work of art from the Palace Theatre Museum, thief Lucas Seyton, the Fallen Angel, stumbles across another robbery in progress.

Inside the den, he finds the body of a European male who appears to have been stabbed to death, but the blood trail which indicates he was killed somewhere other than this room leads straight to a solid wall.

The Tracer directs the TARDIS to Shanghai in the year 1934, but it is giving off very strange readings, and the three segments which the Doctor and Romana have already collected are glowing in an unusual manner.

The Doctor, questioning whether two segments of the Key would be located on the same planet, follows the trace to Nang Tao airport, where they find an abandoned car with blood in the back seat and a knife made of an alloy that has not yet been invented on Earth.

The Doctor soon determines that the Tracer is picking up another source of chronon radiation which is not related to the Key to Time, but since he can't locate the fourth Segment until the interference is dealt with, he and Romana must remain to investigate.

The Doctor studies the stunned bandits and finds that they are all footsoldiers from the Tong of the Black Scorpion and that they are carrying geomantic compasses embossed with the blood of a young woman.

Knowing him to be dangerous, but believing that he can be made to understand why she is doing what she is, she orders Kwok to take him through the Dragon Paths and strand him in the Orkney Islands; by the time he returns to Shanghai by more conventional means, her plan will be complete.

Li returns to the mansion to arrest Hsien-Ko but is attacked by the thing she stole from the Palace Museum, the killer automaton known as Mr Sin, and barely escapes with his life.

Woo has heard Kwok's name mentioned in a conversation about the movement of a Japanese brigade, and the Doctor takes them to question the colonel in charge, only to be recaptured by Hsien-Ko.

If Hsien-Ko focuses a nuclear reaction through the telluric currents, she will short out the zygma beam and release fifty years of accumulated energy—enough to use the Dragon Paths to travel anywhere in time and space.

Hsien-Ko has promised the Tong that she will restore their god to them, but she admits to Romana that she knows the truth; Magnus Greel is a madman who deceived her father, and it was only due to the Doctor's intervention that Chang died an honourable death.

As the reactor control room begins to fill with radioactive steam, many of the guards turn on each other, and a battle breaks out between members of the Tong, who no longer understand what is happening or who can be trusted.

The Doctor uses the TARDIS to expel the Time Cabinet back into the zygma beam, and it completes its journey but the temporal feedback is shorted through Hsien-Ko's body, and she is incinerated before the horrified Kwok's eyes.

The Doctor uses the TARDIS to bury the reactor under fast-drying concrete so the Japanese will not find it, and he and Romana set off on their quest once again, as Hsien-Ko's death means that the chronon interference has cleared up.