When little monster Lucy falls through a door to the human world and goes missing, Sully and Mike know they must find her fast before she comes to any harm. But the smart and curious Lucy has other plans: to uncover the mystery of human energy once and for all. As Mike and Sully follow in her footsteps, she finds herself on a learning adventure that takes her all over the world. Will Mike and Sully catch up with Lucy and bring her back to Monstropolis safe and sound?