Lurking discreetly somewhere off the north-eastern coast of Scotland, a Royal Navy submarine waits to receive top-secret signals and relay them to the mainland.
Ordinarily, operators in its cramped radio room might be listening out for vessels from Russia's mighty Baltic Fleet ploughing through the North Sea.
But the 'intercepts' they anticipate today couldn't be further removed from those they usually pick up while playing seaborne cat-and-mouse with foreign powers.
Delivered in a South London twang punctuated with expletives and raucous laughter, they are the bugged private conversations between five young men, holidaying 30 miles away in the Highlands.
And though their chatter might seem banal, it could be of enormous importance — not to the UK defence department but to murder squad detectives at Scotland Yard.