Brothers, Charlie and Freddie White, along with their friends--The Rainbow Gang--find a very unusual ancient looking chest in their dad's garden shed. The chest magically speaks to them and sends them underground to a world inhabited by elves. The elf world is being disturbed by a clumsy young giant from a world below the elf world. The ensuing, sometimes humorous, adventure brings them into contact with lots of unusual creatures and sees The Rainbow Gang set off on a mission to rescue the young giant, whose actions have disrupted the water tables underground. During their adventure, the gang encounters giant hedgehogs, giant moles and large talking fish. Charlie and his friends come into contact with another gang--The Girls--who help them in their adventure and both gangs strike up a lasting friendship.