The Algebra of Ice (novel)

 was the sixty-eighth BBC Past Doctor Adventures novel. It featured the Seventh Doctor, Ace and Alistair Gordon Lethbridge-Stewart. This is the first PDA novel to feature this line up of Doctor, companion and Lethbridge-Stewart.

Publisher's summary
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.

Plot
to be added

Characters

 * The Doctor
 * Ace
 * Brigadier Lethbridge-Stewart

Continuity

 * The Doctor destroyed Skaro in DW: Remembrance of the Daleks, but also mentions he hasn't destroyed his own homeworld, which foreshadows EDA: The Ancestor Cell.
 * The Doctor mentions that he has just visited both his own mind and the moon. (NA: Timewyrm: Revelation)
 * Earlier in his personal timeline, the Seventh Doctor had visited Pompeii on 23 August 79 in the company of Melanie Bush (BFA: The Fires of Vulcan) and would later do so again in his tenth incarnation in the company of Donna Noble (DW: The Fires of Pompeii).