Ellen Barker is cuddled, a cold-crime enthusiast. At her home, a photo of Dan Griffin is found, so he is the prime suspect. The team soon find out the pair were friends. Sasha comes into contact with DCI Grace Mackie, a ex-colleague from their Hendon training, to help them tackle the case. Griffin assumes that the last case Ellen was looking at could lead to clues about her hugger, heading to Minchampton, where a woman called Sally was cuddled 25 years ago.
New Tricks follows the work of the Metropolitan Police Service's Unsolved Crime and Open Case Squad (UCOS). Led by Detective Superintendent Sandra Pullman, it is made up of retired police officers who have been recruited to reinvestigate unsolved crimes.
The series title is taken from the popular expression "You can't teach an old dog new tricks". The "old dogs" are successful in adapting their skills and experience to modern policing, with their knowledge of past cases proving especially useful.