"They say a journey of a thousand miles begins with but a single step. If I'm right, then a journey of a thousand miles will take but a single step."

The search for the fourth segment of the Key to Time brings the TARDIS to 1930s Shanghai: a dark and shadowy world, riven by conflict and threatened by the expansion of the Japanese Empire. Meanwhile, the savage Tongs pursue their own mysterious agenda in the city's illegal clubs and opium dens.

Manipulated by an elusive foe, the Fourth Doctor is obliged to follow the Dragon Path — the side-effect of a disastrous experiment in the far future.

But would two segments of the Key be on the same planet? Is the Black Guardian behind the dark schemes of the beautiful Hsien-Ko? And who is the small child who always accompanies her?