London in winter, with snow lying heavy on the ground... On this particular occasion, it is the job of Department Z to safeguard the many delegates of those countries who have arrived in England to attend a top-level international conference.

But things start to go wrong. An attempt to assassinate the Russian Virnov from the roof-tops opposite the conference hall fails because the Department received advance warning.

But as Craigie and his men investigate further, it becomes increasingly obvious that they are dealing with a highly professional organisation whose arm extends to the most unlikeliest places...