Donal McCarthy is using small cells to bomb soft targets in Britain. After a number of doorstep bombings three members are caught thanks to computer expert Freddie Ross. Cmdr. Allenby decides Ross should transfer to Belfast.
Following the capture of an IRA cell, in London, Duffy and Scanlon are in hiding. Kevin Brady computer analysis names Donal McCarthy as the Provo chief. McCarthy is looking for a mole in his outfit while Brady visits the Maze Prison.
Waitress Clare Bennett is tasked to entrap Freddie Ross. Duffy dumps Scanlon's body in the North making it look like the work of the R.U.C but raises suspicious. McCarthy begins a dummy operation to test Duffy's loyalty.
Clare Barrett has doubts about spying on Ross. When youths start rioting she is killed with a rubber bullet. McCarthy is convinced the riot has blown his operation but the organiser is the traitor. Ross believes Clare was murdered.
Ross discovers that Clare Barrett's sister Maureen was meeting with a contact and passing on messages and she was killed by one of her own. Ross and Burke set a trap for McCarthy as both side's race to find the same mole, Dermot Shea.