A night out with his girlfriend goes badly wrong when Raza Shar is arrested and identified as a potential informant for London's Counter Terrorism Special Unit (CTSU). Detective Sergeant Gabe Waters is partnered with new recruit DC Holly Morten and introduces her to the world of informants. Ambitious and uninhibited, Holly pushes boundaries to get Raza on board. Gabe's key contact, Yousef, has information that links to a prolific terrorist - Ahmed El Adoua. El Adoua was responsible for a major attack in Rotterdam, and CTSU fears he might have been in London planning a similar attack. When Yousef goes missing, the pressure is on for the CTSU team to find out more.