By day, Julia Ebner worked at a counter-extremism think tank, monitoring radical groups from the outside, researching, reporting and advising policy makers. But she began to feel that the view from outside only gave half the picture; she needed to get inside these groups to truly understand them.

So she spent two years going undercover in her spare hours - late nights, holidays, weekends - adopting five different identities, and joining a dozen extremist groups across the ideological spectrum, from jihadists and Christian fundamentalists, to white nationalists, conspiracy theorists and radical misogynists.

Her journey would take her from a Generation Identity global strategy meeting in a pub in Covent Garden, to a Neo-Nazi Music Festival on the border of Germany and Poland. She would get relationship advice from ?Trad Wives' and Jihadi Brides and hacking lessons from ISIS. She was in the channels when the alt-right began planning the lethal Charlottesville rally, and was confronted by Tommy Robinson, a camera and his followers as he stormed her workplace.

In Going Dark, Ebner takes the reader on a deeply compulsive, terrifying, illuminating journey into the darkest recesses of extremist thinking, exposing how closely we are surrounded by extremist thinkers every day, the changing nature and practice of these groups, and what is being done to counter them.