Mark Stone began with an undercover law-enforcement mission in 2002. Scotland Yard’s intelligence service was concerned about a growing trend of environmental protests in England, and in a controversial move, they decided to put a man on the inside that could give them early warning on upcoming actions. They chose Mark Kennedy, an officer coming off the undercover drug beat. They gave him a new name, changing Kennedy to Stone. Kennedy grew his hair out into a ponytail, got piercings and tattoos, and started staking out protest groups. As Stone, he became popular in the movement. He protested a dam in Iceland and fell in with anarchist groups in Germany and Italy. In an action at England's Drax Power Station, he was beaten by riot police who had been called in based on his information. They broke his finger and dislocated a disc in his back.

In 2010, the ruse fell apart. His girlfriend found a passport bearing the name Mark Kennedy, and little by little, the threads came loose. In January of 2011, The Guardian published his picture, setting off a massive scandal and forcing Kennedy to give up on Mark Stone for good. In an interview with the paper, he described the eight years of undercover work as an existential ordeal. "I know I have these core values," he said, "but it's going to be a long process to find out who I am." With that, he seemed to have left Stone behind.