Raza, a young, second generation Pakistani man from East London, is coerced by Gabe, a Counter-Terrorism officer, to go undercover and inform for him. Gabe, who has a past he is unwilling to expose, is joined by Holly, his new and ambitious partner whose endless curiosity becomes threatening to him. As the central counterterrorism investigation heats up, the stakes for all three get higher and higher. Informer tells a story about identity in a world where lines are increasingly being drawn and sides are being taken. What happens when you, or your friends, family or neighbours fall on the wrong side of that line and the personal becomes political?
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.
Gabe needs Raza to get closer to Dadir, the brother of Yousef, his missing informant. A nervous Raza struggles with his new role as he befriends and spends time with the unpredictable Dadir and his Bridge Town Crew. Gabe and Holly discover Yousef has been murdered - could Big Shot, the London contact linked to El Adoua, be behind it? The pressure on CTSU to find out whether a London attack is planned mounts, and Gabe pushes Raza further to find Big Shot, but instead of invading Dadir's life, Raza finds Dadir invading his.
The toll of Raza’s double life begins to weigh heavily on him as he attends Yousef’s funeral seeking information. CTSU continue to follow all leads; an unexpected tip from Raza leads them to follow an Albanian Crime family, the Gramos. Meanwhile, Gabe runs into a ghost from his past and his behaviour piques Holly's interest. Noticing a change in his older brother, Nasir follows Raza and inadvertently makes a new friend on the Bridge Town Estate. Raza struggles to hold onto his old self when he accompanies his girlfriend to her sister’s wedding.
The terrorist cell is exposed and the final tragic details surrounding the attack on Café 66 are revealed. The fallout from these catastrophic events and revelations that follow change the lives of all those involved forever.