US Federal agent John Dennison arrives in the UK to meet Chief Inspector Philip Hunter of Scotland Yard, and to find $300,000, smuggled ashore via Liverpool. According to Dennison, the smugglers are intent on buying diamonds, to ship back to the US.

Two suspects are followed to a London hotel and the diamonds are found. Shockingly, they are discovered to be real . . . but synthetic.

The diamond deal, involving an eccentric middleman, exposes a much bigger racket. Hunter and Dennison have a complex plot to unravel, some dangerous men to deal with - and Dennison the beautiful, red-headed Martine to keep an eye on . . .