Inspired by his own career as an SAS soldier, Andy McNab's best selling series continues with Crisis Four as Nick Stone finds himself at the centre of a deadly game of cat and mouse when he is ordered to track down a rogue colleague.

Nick Stone is extremely highly trained. Clever, ruthless and very effective, it is no surprise that he is hired by British Intelligence. On deniable operations - one of the most dangerous lines of work. Sarah Greenwood is beautiful, intelligent and cunning - and the only woman Stone has ever truly opened up to. But now he has been ordered to hunt her down.

Stone must get to the heart of a terrifying conspiracy theory to which only Sarah holds the key. But will he manage to before the tension reaches boiling point?