The pilot episode, "Killer", broadcast in 1983, introduced the character of Detective Chief Inspector (DCI) Jim Taggart (played by Mark McManus until his death in 1994), a tough and experienced detective who had worked his way up through the ranks. His original sidekick was Detective Sergeant (DS) Peter Livingstone (Neil Duncan), who represented the new breed of young graduates entering the police force. Taggarts' boss was Superintendent Jack McVitie (Iain Anderson), nicknamed the biscuit as a play on his surname and a popular brand of biscuits.

In 1987 the character of Mike Jardine (James MacPherson) was introduced, and when Neil Duncan left the series in 1989, his character was replaced by a new female sidekick, Jackie Reid (Blythe Duff). Jardine became the central character following McManus' death in 1994. Much was made of the platonic relationship between Jardine and Reid, with the two being shown to share a brother/sister-like bond.

When MacPherson left the series in 2002, his character was killed off, and replaced with DCI Matt Burke (Alex Norton, who had previously appeared in the series playing a murder suspect in the 1986 episode, Knife Edge), though by this time the series had become more of an ensemble piece, following the addition of Colin McCredie as DC Stuart Fraser in 1994 and John Michie as DI Robbie Ross in 1998. Alex Norton's Matt Burke character has gained widespread critical acclaim, many believing him to be the first credible successor to McManus's original Taggart character.