After some illegally imported caesium-137 is stolen, Nate is recruited by Angela Zamparo, a fellow FBI Special Agent, who suspects the involvement of white supremacist groups.

Through Zamparo's connections, Foster is introduced to a small group of Neo-Nazis led by Vince Sargent, a local leader who is familiar with their prime suspect, alt-right talk radio host Dallas Wolf.

With Sargent's introductions, Foster becomes ingratiated in the movement and meets Andrew Blackwell, the leader of a premier white supremacist militia, as well as gaining Wolf's attention by convincing him he can fund an expansion of his radio show.

After earning Blackwell's trust by saving him during an attack on a white power rally by antifascists, Foster is brought to a crude military complex operated by his militia.

Blackwell is meanwhile also dismissed as a possible threat, as he appears to use the D.C. water network plans as a way to lure in potential recruits with promise of participating in an impending terror attack.

Ragussis based his story on the experiences of FBI agent Michael German, who spent a year undercover with white supremacists.

[3] On July 30, 2015, it was announced that Radcliffe had been cast in the lead role, portraying a young FBI Special Agent who goes undercover to find and stop white supremacists trying to make a dirty bomb.

The site's critical consensus reads, "The unsettling Imperium boasts troublingly timely themes and a talented cast led by Daniel Radcliffe as an undercover FBI agent infiltrating a ring of white supremacists.