Elton would be a juvenile dog; too old to be a puppy, but not quite an adult yet. He has spent his whole life in a nice, suburban home with a wonderful suburban family. Days are spent chasing birds and cats and squirrels in the backyard, and evenings are spent lying at the foot of his master (a teenage boy)'s bed eating dinner scraps. This is all Elton wants in life-food, sunlight, and his family-he can't imagine anything else.
Unbeknownst to him however, big changes are happening within his family. His master is eighteen years old and about to leave home for college. The parents are in the middle of a divorce. And to top it all off, the family has put the house up for sale. One day, while Elton is accompanying his master on a grocery trip, he sees a squirrel from the window of the car and leaps out. He leaps out and chases the squirrel past the grocer's, past the barber's, past the town square, and the familiar neighborhood. By the time he loses interest in the squirrel, he is thoroughly lost.
The rest of the story would revolve around him trying to find his way back to his family with the help of a few new friends that he makes along the way. I don't quite have the details of the journey yet, but idea is that he'll see/experience things that he's never been able to see/experience outside the comfort of his old home. Through this, Elton comes into his own; he develops confidence, learns to accept himself, and creates an adult identity. The biggest part of that new identity is his discovery-and subsequent love-of rock music. His family, while wonderful people, were very much conservative in their music tastes. They played classical music in their homes, and so Elton was never exposed to rock music until he ended up lost. When Elton hears rock music for the first time, he is fascinated, but he is inexperienced and too timid to play. By the end of the story, he has developed the confidence to play a song onstage with his friends. This represents Elton's self-actualization.
At the very end of the story, he will have made it back to his old house. He runs up to the window, excited to see his old family and to get back to his old life, but instead, there's a completely new family in his house. And for that matter, the house has changed as well. It looks smaller than he remembers, and all his favorite spots have been replaced. Elton takes one last, long look at the house. He smiles, remembering his life there, and silently wishes his old family well. Then, he walks away to join his friends again.
The major theme of this story is growing up. It is about venturing away from the safe space of childhood, and about developing one's own individual identity. It is also about realizing that childhood isn't really something that you get to go back to. The elements that make up one's childhood change- just like each person changes-over the years, and what somebody had at one point won't necessarily be there. The only thing to do is to move forward.