Scott Turner, an obsessively neat police detective, acquires Hooch, a large and slobbery French Mastiff, after the murder of Amos Reed, a local junk yard owner. Turner is set to transfer to a better job with the Sacramento Police Department, and Detective David Sutton is to be his replacement.
However, Turner pleads with the police chief to let him take on the Reed murder case. Believing that Hooch is the only witness he has to the Reed murder, Turner brings him home. The energetic dog promptly destroys Turner's house, his car, and turns his life upside-down. On a positive note, however, Hooch also instigates a romance between Turner and the town veterinarian.
Eventually Turner, with the help of Hooch, uncovers a money laundering operation led by the police chief, and Hooch gives his life to save his master. The police chief is also killed. In the end, Turner becomes Police Chief, and Sutton does indeed take Turner's former position. On the home front, Turner and the vet are married and expecting a child. In addition, the vet's collie has given birth to half a dozen puppies, including one that looks and behaves uncannily similar to Hooch.