Oleg and Emma entered their den to find a cardboard spaceship standing exactly where they usually sat. Slowly, the front door opened and out stepped a boy. 'My name's Sebastian Cole,' he said. 'But you already know that.'
When Oleg and Emma invent a new classmate called Sebastian, they are amazed when he appears - very much real - in their secret den.
Sebastian isn't like the rest of their classmates. He's never eaten pizza, he's not sure what goose bumps are, and he has a satchel that seems to hold an endless supply of hot ice cream.
But as the trio begin their adventures, more impossible things keep happening, from a runaway goat appearing at school to a sighting of some snowwomen