Parker J. Cloud, a former raincloud, his friend Angus, and pet walrus Russell deliver mail all across Mount Middlemost.

Parker J. Cloud, an exuberant rain cloud, lives and works in the Middlemost Post Office with Angus, a burly, rule-abiding mailman who cherishes his solitude and values efficiency, and Russell, their magical pet walrus whose stomach is so big that it doubles as a storage room. Together they journey through Mount Middlemost and the six territories of Somewhere to deliver the mail, while meeting some exceptionally quirky and peculiar people along the way. But as this loving and unconventional trio grows closer, they remain blissfully unaware that they have been brought together by one fateful stormy event.