It's been over 40 years since RAF Excalibur disappeared on its war time secret mission. Unbeknown to anyone connected with the mission at the time the plane had been shot down shortly after take off. Its resting place is at the bottom of Lake Laffey where it stayed along with its crew and Philip Graves biological father, Squadron Leader John Laine. Laine would return within days of his death at a local seance to warn of the latent danger posed by his plane's cargo, only for the authorities to intervene and discredit the medium. Now 40 or so years later and with the impact of local earth tremors nearby, the wreckage of the Excalibur is breaking up and with it the liberation of gas, its cargo bound for Germany. The gas appears across Lake Laffey as a mist drifting inland and affecting adversely, members of the local population. This was its purpose 40 years earlier only the target then was the German air force. A number of inexplicable crimes follow perpetrated by people acting completely out of character and with no memory of their actions afterwards. The prime suspect of a murder accuses a prominent ex-minister of the crown of being complicit in the crime under interrogation, but the man concerned is bed-ridden and aged. The public embarrassment though is enough for MI5 to call on the help of New Scotland Yard Inspector Philip Graves based on his reputation. As Graves delves into the past of the suspect and the ex-minister, involving his own visit to a medium, he begins to unravel the past and the whereabouts of the ill-fated Excalibur, and that of his own connection to John Laine. This is not before a number of other crimes befall the residents who live close to Lake Laffey.