A counter-terrorist police raid kills an innocent man after mistakenly raiding the wrong flat. DS Steve Arnott refuses to take part in a cover-up. He is transferred to AC-12, an anti-corruption squad led by Superintendent Ted Hastings. Hastings informs Arnott that his first investigation will focus on DCI Tony Gates, the senior officer of TO-20, a unit of Central Police CID, who has just been made Officer of the Year. Hastings has received a tip-off that Gates has a suspiciously high clearance rate due to "laddering" – a process in which a number of different charges are placed on the same defendant to increase an officer's number of successful cases. Initially, Hastings interviews Gates on a minor charge – failure to report accepting a free breakfast for apprehending a mugger outside a local cafe.