Tom Marcus - not his real name - was a spy; an undercover agent for the British security agency, MI5. For several months, he pretended to be a homeless man living on the streets of London. He went to great lengths to blend in, and it was all worth it when he ended up preventing two coaches full of school children from being blown up. He gave this interview in January 2017.

Tom Marcus has written a book about his experiences, it's called I Spy: My Life in M15.

Get in touch: outlook@bbc.com

Presenter: Jo Fidgen
Producer: Katy Takatsuki

Picture: A homeless man in the streets of London
Credit: Dan Kitwood/Getty Images