Lou first appears as a puppy along with other beagle puppies in a farm. While all the other puppies seek a comfortable home, Lou seeked adventure. Lou tries to jump out the window but fails, and the other puppies laugh. Soon, D.O.G. sends in agents to get a home after receiving the news the Brody family was adopting a puppy there.
When Lou was picked due to a mistake, Butch was sent to train him and partner with him.
In the end, at a Christmas tree factory, he defeated Tinkles and chose his family over being an agent. Surprisingly, he appears as leader of D.O.G. in the second film.