Good Boy follows Todd, a man suffering from a chronic lung illness, who leaves New York City to move into his late grandfather’s long-vacant rural home in the woods, bringing along his loyal Nova Scotia Duck Tolling Retriever, Indy. The house has rumors of haunting, but Todd hopes the isolation will aid in his recovery. Indy, however, immediately senses that something is deeply wrong: phantom shadows, strange noises, and an invisible presence that seems to threaten both the house and Todd’s fragile health.

As Todd’s condition worsens — persistent coughing, blood, and growing confusion — his behavior becomes erratic. Indy witnesses unsettling visions: a skeletal, muddy figure, spectral manifestations of the previous dog, Bandit, and Todd’s grandfather’s appearance in dreams and VHS tape recordings. Indy’s perceptions blur the line between supernatural horror and what might be real or imagined. At one point Todd isolates Indy — chaining him outside in a doghouse — yet the dog’s loyalty never wavers.

In the climax, Todd succumbs to the illness, and the dark entity seems poised to claim him. Indy breaks free from constraints, navigates the haunted house, and confronts the entity, but is powerless to save Todd. The next morning, Todd is found dead. Indy, grieving, pauses at a distant whistle, then ascends into the world with Todd’s sister, Vera, suggesting that even in loss, the bond persists.