Someone at a large British armament company is selling Britain's military secrets. To learn who, MI5 calls again upon Max Sargent to go undercover at the firm, dangling the designs of a new long range sniper system as bait. But MI5 is not just after who is doing the selling; it wants who is brokering the deal as well as who the end buyer is which means that Sargent must not just play detective - he must play the traitor.