This is the second book in the Spectrum Series, following on from, 'Thirteen - Violet.'
Tilly's friend Isabella decides she no longer wishes to be a Thirteen. In attempting to help her the Thirteens become involved with an alien and icy planet, Ussar, and its inhabitants.
They meet polar bears, robots and the White Riders. They learn more of ancient prophecies.
London and the Delft are attacked.
Tilly has to make a momentous decision.