The book is at its most vivid, however, when conveying the sparse but gripping details of Oggins's heroic refusal to confess during a year of interrogation in the Lubyanka prison, the months of his horrifying journey to the frozen camp at Norilsk and his time in what must surely be one of the nadirs of human misery. And then the end, in the white room in the building which now houses the exclusive medical centre for the KGB's successor.

Oggins was told it was a standard pre-release medical examination. Instead, it was an injection by Professor Mairanovsky of curare, a resin used by South American tribes to paralyse their prey. Death was excruciating and took 10-15 minutes, with the victim conscious but unable to shout or move. The purpose was to see whether the poison was traceable after death. It wasn't, so the experiment was a success.

Which is more than can be said for the other experiment to which Oggins devoted his life. Loyal to the end, he deserved better of his masters as did the millions of others oppressed by the cause he served. Yet it is hard to feel great sympathy for him; blindness can be culpable.

In some ways, this book represents the non-fictional counterpart to Arthur Koestler's seminal novel of the period, Darkness at Noon.