See Spot Run
Agent 11 is a crime-fighting bullmastiff used by the FBI; he partners with his master, Murdoch (Michael Clarke Duncan), in Seattle. As they go after the Mafia boss, Sonny Talia (Paul Sorvino), Agent 11 attacks him and rips off one of his testicles. He is taken to the hospital to have it surgically replaced. As payback, he sends his two bodyguards, Gino and Arliss (Joe Viterelli and Steve Schirripa respectively), out to kill Agent 11. For protection, he is sent to a training facility in Alaska.
After Agent 11 escapes, he meets Gordon Smith (David Arquette), who works as a mailman. Gordon dislikes dogs and has volunteered to take care of James (Angus T. Jones), the young son of his neighbor, Stephanie (Leslie Bibb). Gino and Arliss have continued their search for Agent 11, whom James names "Spot." He doesn't want to catch balls or frisbees because Murdoch told him not to play when he was a puppy, but he eventually starts to play with James and Gordon like a normal dog. Later, Gino and Arliss try to kill him while he is with Gordon and James at a pet store, but he outsmarts them.
When the FBI finds out that Spot is living with Gordon, they take him back. He escapes and finds Gordon and James again. Sonny returns and attempts to kill him, but is outsmarted, captured and imprisoned.
The FBI agents try to take Spot back, but in the end they decide to let him choose with whom he wants to live. He chooses Gordon and James, although he gives Murdoch a goodbye lick. Stephanie returns and is very upset with Gordon for what had happened, but James convinces her that Gordon is a good guy, and they end up together.
One of the film's last scenes is of Sonny in prison, his testicles have been replaced by metal balls (the other one having also been bitten off by Spot) that constantly clack together. One inmate calls him "Music Man", and he threatens, in a higher voice, "I'm gonna catch you in the yard. We're gonna settle this man to man!".