In the first of two programmes, Peter Taylor looks at the real world of modern spies. For the first time on television, serving British secret agents talk about their work - from an MI6 agent runner to an MI5 surveillance officer. This time, Modern Spies investigates how today's spies are recruited and probes the secrets of spycraft, from the sleeper cell to the brush pass and the cut out to the cyber spy.

In the second of two programmes, Peter Taylor investigates how far today's spies can go to prevent terrorist attacks. When does an undercover operation cross the line and become entrapment? Were British spies complicit in the rendition of terrorist suspects in Libya, where they were allegedly tortured? Do spies ever have a license to kill?