They are young, all-American girls who enjoy horse riding, karate and Sherlock Holmes. But there's more to Brynne, Tess and Savannah than wholesome pursuits - they're exorcists.  The girls believe much of the world's population is possessed by evil spirits which are causing addiction, depression and suffering. In a fight against the devil's army, they have been touring America performing public exorcisms on their believers.  Now they are taking the fight to a city they think of as one of the most spiritually corrupt in the world - London. But what will Brits make of these evangelical American exorcists?