Detective Michael Dooley leaves his squad car during a stakeout to contact his girlfriend Mel Harris Tracy, but a helicopter attack destroys the vehicle and upends his routine. At the station, he confides in his lieutenant, Ed O'Neill Byers, that the assault probably comes from Ken Lyman, the drug lord Dooley has hunted for years, yet he insists on working solo despite Byers’ pleas for a partner.
Following a tip from an informant, Freddie, Pruitt Taylor Vince Freddie, Dooley learns that Lyman ordered the attack and uncovers a lead about a sprawling warehouse where the drugs are stored, though finding it proves difficult in the vast space. After aiding in a delicate drug bust, Dooley asks Brannigan to lend him a K-9 to help locate the stash, and Brannigan brings in an experienced but quirky German Shepherd named Jerry Lee.
Dooley and Jerry Lee head to the warehouse, but the search initially yields little. The dog initially ignores Dooley’s commands, and only a marijuana joint turns up. Later, at a pub where Dooley surveils a go-between, Benny the Mule, Jerry Lee proves his worth by saving Dooley when the undercover setup is blown. With the dog's help, Dooley subdues Benny and learns the location of Lyman’s next shipment. Meanwhile, Lyman kills Freddie and orders his henchman Dillon to finish Dooley off before the shipment arrives.
Back at Dooley’s apartment, Jerry Lee steals the scene as Tracy accepts Dooley’s story about saving the dog from the streets. The next day, the reunited pair bond over meals and surveillance of Lyman’s operations. They come under fire, nearly killed, and chase the attacker to an empty building; the suspect tumbles to his death. In the man’s car, a clue points to an auto-dealer shop, where Jerry Lee identifies a red Mercedes loaded with drugs, revealing that Halstead, the dealership owner, works for Lyman. The canine companion’s antics extend to a lighter moment when he falls for a poodle, much to its owner’s chagrin.
Returning home, Dooley discovers Tracy has been kidnapped. Infuriated, he crashes a party at Lyman’s mansion, prompting the police to arrest him—an intentional move to mislead Lyman into rushing the shipment, though Byers becomes involved in the fallout. Using Jerry Lee’s flatulence as a diversion, Dooley escapes with the dog. He explains to Jerry Lee how he met Tracy, then spots a truck driven by Halstead pulling a trailer with the red Mercedes and gives chase, resulting in an arrest.
The pursuit leads to the desert, where Lyman, with Tracy hostage in his limo, awaits. Dooley uses a videogame as a makeshift detonator to taunt Lyman into surrendering Tracy or risk the truck exploding and drawing the ire of the buyer, Gilliam. The ruse betrays Dooley’s cover, sparking a firefight in which Dillon is killed and Jerry Lee pursues Lyman. Unable to outrun the dog, Lyman is shot by Gilliam’s henchmen, but Dooley and Tracy rush Jerry Lee to a hospital, where a surgeon fights to save the dog. In a late-career moment, Dooley delivers a eulogy to Jerry Lee—only to discover that the dog will live.
After a period away from the force, Dooley, Tracy, Jerry Lee, and a fluffy poodle take a well-earned vacation together in Las Vegas.
