Imagine that mathematics and magic are the same thing. Also imagine that all those people in madhouses in Lovecraft, H.P. stories are right and there are other universes where ancient malevolent entities are just waiting for an invitation to visit for a quick massacre of humanity before breakfast...
...plainly all nations would have a secret agency dedicated to protecting the public and keeping them blithely unaware of the outrageously dangerous world they really live in.
Enter Bob Howard, junior occult spook in the Laundry, Britain's most secret and most magical intelligence agency, armed with a palm pilot and a pigeon's foot, ready (OK, not very ready) to protect us from whatever sinister forces are abroad, whether they be terrorists, possessed people or Nazis from another dimension.
Great fun if all this nonsense sounds fun to you, probably really bad if it doesn't.