Carl is an earthworm. He spends his days happily tunnelling in the soil until a field mouse asks him a simple question that stops him short: "Why?" Carl's quest takes him on an adventure to meet all the animals of the forest, each of whom seems to know exactly what they were put on this earth to do, unlike the curious Carl. But it's not until the world around him has changed that Carl begins to realise everyone, no matter how small, makes a big difference just by being themselves.