Working under the hot urban environments of Hong Kong, the investigators use scientific evidence obtained from the forensic scientists to solve crimes.

The two work closely with Senior Inspector Nicole Leung (Yoyo Mung) and Sergeant Sum Hung (Raymond Cho) of the Kowloon West District Crime Squad.

In the sequel, two new characters, science evidence officer Ivan Yeung (Kevin Cheng) and Senior Inspector Isabella Ma (Charmaine Sheh) were added to the cast.

The two work closely with Senior Inspector Ada Ling (Kate Tsui) and Sergeant Lee Chin-fung (Ron Ng) of the Kowloon West District Crime Squad.

The three work closely with Senior Inspector King Sir (Shaun Tam) and Sergeant Ko Ching (Rebecca Zhu) of the Kowloon West Regional Crime Unit.