Third grader Mortimer Henryson wants to bring a plant home from school for the summer, but he doesn't want just any plant. The only plant for Mortimer is Plantzilla. At first, his parents are happy to have Plantzilla come live with them. But when Mortimer finds Plantzilla's "growing preferences" include a diet of people food (mostly meat), they become a bit concerned. And when their beloved chihuahua suddenly goes missing, Mortimer's mother and father start to wonder whether the family can last an entire summer with Plantzilla.