Two children make up an imaginary friend when they are asked to write an essay in class, and are amazed when he actually comes to life.
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 ice cream.
But as the trio begin their adventures, more impossible things keep happening. They are soon chased by the mysterious Institute of Unreality, who want to capture and erase Sebastian, to restore order to the world.
Can Emma and Oleg protect their new friend and keep the magic of the impossible alive?