There are two stories working here. Someone on a mountain bike uses a crow bar to assault people as they pass through an underpass. The second, and the more extensive story, has to do with a hoodlum who runs a dogfighting ring. When Frost investigates, they arrest everyone and free the dogs. However, the mastermind isn't there. He has a way of getting away with things. He is, at best, a disgusting creature. He has involved his son in his activities and mentions that a young man may have blown the whistle on their activities. The son and a friend decide to beat this kid up, but end up stabbing him to death. The father, an intimidating figure, begins a process of intimidation to get his son and the friend freed. A woman at the RSPCA becomes friendly with Frost after she ID's the two kids in a lineup. She and Jack then become targets of the bad guy and his henchman. Her dog shelter is set on fire and she is nearly killed. This episode sets things up nicely. We are privy to the horrible guilt of the young men who dare not tell their story because their prospects would be ruined.