Tom Hanks plays Scott Turner, an obsessively neat police investigator, who acquires Hooch (Beasley the Dog), a large and slobbery Dogue de Bordeaux, after the murder of Amos Reed (John McIntire), a local junk yard owner he was friends with. Turner, bored with little police work in Cypress Beach, California, is set to transfer to a better job in Sacramento, and fellow investigator David Sutton (Reginald VelJohnson) is to be his replacement. However, Turner pleads with police chief Howard Hyde (Craig T. Nelson) to let him take on Amos' murder case. Believing that Hooch is the only witness he has to the 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 new town veterinarian Emily Carson (Mare Winningham).
Eventually Turner, with the help of Hooch, uncovers a money laundering operation led by Chief Hyde, and Hooch gives his life to save his master. Hyde and his partner are also killed. In the end, Turner becomes Police Chief, and David does indeed take Turner's former position. On the home front, Turner and Emily are married and expecting a child. In addition, Emily's collie Camille has given birth to half a dozen puppies, including one that looks and behaves uncannily similar to Hooch.