When graffiti appears around London by an unknown person claiming to have murdered a celebrated street artist, the team reopens the four-year-old case. Originally part of a gang, the victim went solo when he received support from a wealthy art patron, leaving the detectives suspicious of his resentful former crew members. Meanwhile, Brian signs up to Twitter to chart the gritty life of a lawman, much to his colleagues' displeasure. (thetvdb.com)