PC Sam Harker has infiltrated a group of animal rights activists, thanks to Steve Burton, an old school friend from Liverpool. The group's leader, Ged Mellings, is suspected to be planning some terrorist action. While undercover, Harker takes part in some protests and animal liberation actions, and begins to sympathise with the cause. Despite some misgivings, Harker proves himself to Meddings, and discovers his plan to firebomb a local department store. CID lie in wait and catch the gang planting the bombs, and Harker is distraught to have betrayed his friend Steve.