|Series 3, Episode 3|
|Air date||1 May 2006|
|Written by||Steve Coombes|
|Directed by||Juliet May|
The team revive a 30-year-old investigation into animal cruelty when a dog is found butchered on Hampstead Heath. The original investigation focused on John Fletcher, who was the press officer for The Human Liberation Front, an organization that was set up to counter the animal rights movement but no longer in existence. They seek advice from James Farlow, the animal control officer at the time, but have few leads. They determine that the killer may actually have wanted to harvest the animal's vital organs. During the investigation, Brian is devastated when his beloved dog dies, prompting the team to get him another. Meanwhile, Jack is feeling ill and is hospitalized with some unknown ailment - maybe it is connected to his new friendship with James Farlow.
- 'Old dogs' is the first part of the same saying as 'New Tricks' comes from - the proverb "You can't teach an Old Dog New Tricks"