On a bright and sunny day, at the doorstep of a certain household, the honest and simple-minded Mr. Samuel Smith is attentively listening to his wife's instructions. She asks him to go to the store to buy some things and, while he's at it, take their dog Rover out for a walk. After receiving the instructions, Samuel and Rover each bid farewell to their loved ones and then head out onto the street together. Mr. Samuel is so focused on the shopping list in his hand that he and the dog end up being flattened into a pancake by a speeding car at a crossroad. An ambulance rushes to the scene, but the careless paramedics mistakenly give Samuel and the dog the wrong blood transfusions. Afterward, Samuel begins to exhibit dog-like traits, while Rover starts behaving like a human. The world is thrown into chaos...