One Saturday morning John wakes up to find an invisible something sitting on the end of his bed. The something seems to be gigantic but John can't work out just how big it is. He tries to measure it, but the invisible giant won't let him.
When John asks this strange new friend its name, the letters IVO appear on his mirror, so John decides to call him Ivor. The invisible giant plays tricks on John's mum and dad and gets up to lots of mischief, especially when John takes Ivor to the park and to school.