Kit Hendry has several criminal convictions, but now he is given the choice of a twelve-year sentence or a government mission to trace the security leakage from a Dusseldorf installation. Kit can crack safes - he can also speak German.

Under the vigilant surveillance of Gaunt and also of Bernadette, with whom he is infatuated, he crosses the channel and is exposed to a smallpox victim, but the film he secures becomes the final double take . . .