Marmaduke sneaks aboard the cruise ship as he follows Mr. and Mrs. Winslow. They are just talking about how nothing can ruin their vacation when Marmaduke starts barking and runs towards them and runs into the captain.
The captain doesn't want him on board at first but gives him a job to earn his keep. He has to guard a clever ape and keep him out of trouble. Marmaduke begins his duty and the ape wastes no time in getting out.
After a quick chase with the ape finally getting caught by Marmaduke, they become fast friends and when the captain and the Winslows come to check on Marmaduke, they see him playing chess with the ape.