The Algebra of Ice

In 2004, The Algebra of Ice was released by BBC Books.

Cover blurb
A genius maths nerd, a weird webzine publisher, and the Doctor's old ally, the Brigadier find themselves helping the Doctor and Ace solve what should be a simple puzzle: the appearance of a crop circle in the Kentish countryside.

Hardly uncommon. But there are some peculiar features. It's not a circle but a series of square-sided shapes. It's filled with ice. And it draws the Doctor and Ace into a confrontation with a reality right next to zero.

This adventure features the Seventh Doctor and Ace.