They operate from "The Pit," a secret room beneath the ancient stone stables of Whitstable Castle. This team of experts feeds phony intelligence to the Germans. But Captain Janine Carlisle discovers a frightening secret - a fifth column who call themselves the Scavengers of Graveny Marsh have stumbled upon secret Allied plans for a swimming tank - the Duplex Drive Sherman - ready to go ashore in the first wave of the invasion. Carlisle must stop them before the secret can reach German command. The fate of D-Day hangs by a thread with only minutes to spare.