Every day, Britain's Secret Intelligence Services are fighting an invisible war against a multitude of hidden threats. In this new series three former spies put a group of 15 men and women to the test to see if they have the skills and qualities required for that battle. They go through a series of exercises based on the Intelligence Officer New Entry Course: an intense, unsettling psychological experiment run by people who've been through the real thing. This episode explores the moral ambiguity of espionage work as the trainees are plunged into an environment of limited trust. In the Lethal Force exercise, the group are taken to an undisclosed location in the countryside and asked to work out the right course of action when there are no clear options. During Tradecraft, an exercise that takes place in London's King's Cross train station, they are expected to think on their feet when the unexpected happens.