When life is terrible, you make a safe place in your head and live there.
At thirteen Sim is kept imprisoned in his home by a family who hate him and a government which, rightly, sees him as a danger.
When Sim escapes, worlds come together and pull apart, leaving him somewhere entirely different, in a new world where his extraordinary capablities are appreciated and he is loved.
Happy ending? No. Sim's past has left him deeply flawed and when his own world catches up with him, he can't tell good from bad or right from room.
All he knows is that he'll do anything, anything at all to save the world inside his head- and he does.