Deep in the mud pools and jungle forests of Hippoland lives Humphrey the Hippo. He is usually quite carefree compared to other hippos (who can be a bit grumpy at times). But one afternoon, he finds he is faced with a puzzle. Humphrey and his brother Harold are determined to make a cup of tea. But try as they might, they cannot find a key ingredient: milk.

So, they decide to consult their friend, Wilberforce the Wizard, for he knows everything. Wilberforce uses his magic to send the hippos to the land of Humans (even though Harold thinks humans are "mud sausages"). In the land of Humans, the hippos cause all kinds of chaos, involving grocery stores, credit cards and bananas - amongst other things.

But the two hippos persevere and return in triumph, with a carton of milk in hand, to Hippoland. However, their adventure is not over. They will still need the help of Wilbeforce the Wizard and his new mysterious friend, Clara.