In the land of monsters, there becomes an energy shortage. They get their energy by entering childrens closets and using their screams as a power source. Mike and Sully have to work overtime, and when they do, they notice someone leaves a door open, and a child is set free in Monster Land. Children being evil and harmful creatures to monsters, there is a hunt for the young girl, and Mike and Sully have to try their best to keep her safe, and get her back home.
Jason and Maggie Seaver are ready to start a new chapter in their lives. The nest is finally empty, they've put the family home up for sale, and they're preparing to spend their retirement years traveling. But all that's easier said than done as Mike and a very pregnant Carol, each with their own agenda for wanting their parents to stay put, team up to sabotage the move. At the same time, little brother Ben, now a realtor, needs the big sale of his parents' home to please his boss. Meanwhile, Chrissie, an aspiring singer, just wants to get out from under her parents' watchful gaze so she can launch her career as a rock star.