Isaac Thompson and the Knights of Castle School

One day Isaac Thompson is an average 11 year old playing the role of a porcupine in the school play. The next, he is launching his quest into knighthood.
When his little sister, Lily, goes missing, Isaac and his best friends, Max and Emma, set off to find her. Their search takes them to the Underground - a mysterious subterranean realm that exists underneath their school - where they encounter an army of spear-wielding rats, a talking human-sized bat and a thumb-nosed prison warden. But humans who stay in the Underground for too long transform into strange, unpleasant creatures and are forced to work for the horrible Elf King. Can Isaac and his friends find Lily and escape the Underground before it's too late for them to ever return home?