The movie starts with a very tall lanky high school kid in situations where height is a disadvantage. He is bumped as he stoops down to his locker and is the last to be picked in a baseball team.
Marmaduke then explains how he is in that stage right now the tall, ungainly high school kid. Marmaduke then introduces the members of “his” family the dad, Phil, who is working in a pet food store; the mother, Debbie, who loves him enough to surreptitiously leaves treats for him; the elder sister, Barbara, who’s best friend is her cell phone; the brother, Brian, who is always playing video games; the baby sister, Sarah, who is Mamraduke’s best friend and the family cat, Carlos.
All is well and dandy with the family in Kansas, as Marmaduke goofs around with Dad and plays with the kids. Marmaduke shares Mom and Dad’s bed and passes gas as a laugh. Things are going along smoothly when Dad suddenly brings home news that he was accepted as the marketing manager of a popular brand of organic pet food. The bad news is that Dad has to go to California to get the job. However, job perks include a house with a pool and a company car. Despite misgivings, Mom agreed to the change; the two elder kids are more reluctant. Marmaduke and Carlos watch an episode of “the O.C.” to have an inkling to where they are going to move. They didn’t like what they saw.
Moving day and the elder kids still show their reluctance to leave their old lives behind. Marmaduke and Carlos were drugged during the flight they wake up to see a nice roomy house with a good-sized pool. Even the dog door is big enough to fit Marmaduke. Dad tries to smooth things over with the kids when he enlisted Brother to a soccer team and tries awkwardly to give a ride home to sister. The company car turns out to be a fuel efficient-two seater hatchback with the top-down so that Marmaduke can stick his head out.
Dad goes to work and meets his boss, Don Twombly, whose dream to is to market his organic pet food brand across the United States. His brand is already number 1 in California and he has gone through several marketing executives before hiring Dad. One of the perks is to have your dog stay in a Dog Park while you work. Marmaduke is now in the equivalent of high school.
Marmaduke is greeted by Mazie, a former rescue dog. She introduces the members of her group Giuseppe and Raisin. Mazie also walks Marmaduke around to meet the other “cliques’ the jocks, the drama club and the top dogs. The top dogs are headed by Bosco whose girlfriend is Jezebel, Twombly’s collie. Bosco asserts himself as top dog and Marmaduke has “puppy love” feelings for Jezebel.
Mazie invites Marmaduke to a party where he shows off some moves. Unfortunately, he also crashes the video system. Marmaduke then invites Mazie and the group to have a peek at the party thrown by the top dogs. Some of the dogs in the party has captured a cat and are taunting it as a sport. Marmaduke doesn’t stand up to Bosco and the group leaves the party.
Carlos was bamboozled by Marmaduke to crash the Dog Park. The ploy succeeds, with Marmaduke being accepted (somewhat) by the top dogs but Carlos was miffed as he had his tail bitten as a souvenir.
Meanwhile, Dad sells the idea of having a dog surfing competition. Marmaduke and Mazie have a tender moment as Mazie describes her idea of a perfect date. Before Marmaduke was able to tell how Mazie how pretty a certain body part was, Marmaduke was abruptly taken home by Dad who was excited by his plan.
As dog owners line up the beach, Bosco walks up to Marmaduke and claims that the surf is his. The two are paired up and are up against a giant wave. Marmaduke crashes over Bosco seconds after the wave hits them and Bosco is wiped out. Marmaduke continues his amazing ride that pleased the judges enough to award him first place. Bosco didn’t like it and he tries to attack Marmaduke onstage. Cornered, Marmaduke gives a huge growl, which frightens Bosco off but also, some of the people, including the judges.
Marmaduke is now top dog as he sits on the “throne” of the Dog Park. However, Mazie and her group are still outcasts and Marmaduke seem to have forgotten how kind Mazie was to him. Marmaduke takes Jezebel to Mazie’s dream date, which further saddens Mazie when she found out.
Dad was forced to go on a “working vacation” with his boss to follow-up the success of the surf competition. Dad takes the family except Marmaduke, as the boss’ wife did not allow dogs in the boat they are going in.
Marmaduke has a house party where the guests get drunk and thrash the house. Bosco crashes the party and find out about Carlos. Bosco reveals all he knows about Marmaduke and Carlos then Bosco leaves taking all the guests, including Jezebel, with him. Marmaduke is left with a thrashed out and no friends.
The family comes home to a thrashed house, the “vacation” obviously not a success either. Dad takes out his frustrations on Marmaduke, who was ordered to sleep outside the house in the rain. Feeling sorry for himself, Marmaduke tries to make amends to Dad before going off. As Dad sleeps over his laptop, Marmaduke files Brian’s unused soccer uniform and Barbara’s cell phone (containing her last message) over his laptop. Marmaduke then returns Mazie’s stuffed toy to her house and wanders off. Marmaduke eventually goes to Chupadogra’s turf. Chupadogra was once a fearsome top dog who is now a dangerous stray. Chupadogra tells Marmaduke his sad story, as he tells Marmaduke to go home, as it is the best place for him is with his family.
Mazie, still miffed with Marmaduke, finds her toy and tries to return it to Marmaduke but meets Carlos instead. Carlos tells her that Marmaduke has run away and Mazie gets her group to look for him. However, Giuseppe and Raisin give up but Mazie keeps going.
Dad wakes up in the morning and sees the pile Marmaduke has left behind. He apologizes to his wife, who feels Dad is spending more time at work then with family; to Barbara, who feels bad about missing a party as she was forced to go with the “vacation”; to Brian who never attended soccer practice; then to Marmaduke, who Sarah announces is missing.
The whole family gets into the car and rides around looking for Marmaduke. Barbara gets on her cell phone to alert her friends. Dad turns down an urgent meeting with the boss and gets fired in return. The family runs up to a traffic jam where the street is collapsing due to a busted water main. Marmaduke and Mazie finally meet near the same street where Mazie falls in with the collapsing street. Marmaduke follows her in, as he is spotted by Brian from the car. Marmaduke helps Mazie in the sewers as rescue teams zoom in. Mazie was rescued but Marmaduke was unable to old on due to the enormous amount of water. Dad follows the sewers and sees Marmaduke in the water. Dad jumps in and rescues Marmaduke and the rescue teams haul them in.
The whole family comes together in the rescue van. Dad finally says sorry to Marmaduke and reiterates that he considers Marmaduke as part of the family. Brian’s skateboard buddies tell him that the video of Marmaduke’s rescue was already uploaded to Youtube. Sarah’s hunk boyfriend comes over and reintroduces himself to Dad. The whole family now wants to stay in California, even as Dad hints they might move back to Kansas if that is what the family wants. Marmaduke sees Mazie in another rescue van and share some feelings.
Dad gets his job back as Mr. Twombly is a big pet man he approves of people leaving work to spend time with family, pets being included as family. Marmaduke reasserts himself in the Dog Park without having to be a member of the top dogs. He is surprised to see a new look Mazie, and finally tells her he thinks her eyes are very pretty. As Bosco gets scared off with a bee, Dad and the boss hits on the idea of dancing dogs for the next pet food commercial.
As the dogs dance around the dog park in the commercial, Mom and Dad watch it in their bed, with Marmaduke, as usual. As the happy parents share a moment, Marmaduke again passes gas, ruining the moment.
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