There are things out there, in the weirder reaches of space-time where reality is an optional extra. Horrible things, usually with tentacles. Al-Hazred glimpsed them, John Dee summoned them, HP Lovecraft wrote about them, and Alan Turing mapped the paths from our universe to theirs. The right calculation can call up entities from other, older universes, or invoke their powers. Invisibility? Easy! Animating the dead? Trivial! Binding lesser demons to your will? Easily doable!
Opening up the way for the Great Old Ones to come through and eat our brains? Unfortunately, much too easy.
That’s where the Laundry comes in – it’s a branch of the British secret service, tasked to prevent hideous alien gods from wiping out all life on Earth (and more particularly, the UK). You work for the Laundry. The hours are long, the pay is sub-par, the co-workers are… interesting (in the Chinese curse sense of the word), and the bureaucracy is stifling – but you do get to wave basilisk guns and bullet wards around, and to go on challenging and exciting missions to exotic locations like quaint, legend-haunted Wigan, cursed Slough and Wolverhampton where the walls are thin.
You may even get to save the world.
Just make sure you get a receipt.