Our hero is a freelance journalist called Kenton who has lost all his ready money on poker dice. Then, out of the blue, the only other passenger in his railway compartment, who claims to be a commercial traveller called Sachs, offers him a tidy sum to smuggle a package through customs for him. Kenton is in no position to refuse. He is an honest fellow and goes to the hotel Sachs nominated, to return the package and collect the other half of his fee. Only to find Sachs murdered and Kenton himself the only suspect.
The rest of the book is a series of adventures which reminded me of The Thirty-Nine Steps - Ambler is in many ways the bridge between Buchan and Bond. The Russian agents, Andreas Zaleshoff and his sister Tamara, are by and large the good guys, Colonel Robinson and Captain Maitland, operating on behalf of the London capitalists, are very much the baddies. Smedoff, the ageing femme fatale, who appears right at the end, is a magnificent character.