The kids are having a picnic in the park while Greg is playing with his pet Lionel until they noticed that Lionel has mysteriously disappeared. At night, it turns out Romeo was feeding Lionel some green pet treats that makes him big and calls him Lionel-Saurus though he also has some red treats that makes him shrink. Unfortunately, Gekko keeps on being defensive on his favorite pet despite Lionel's big size which not only makes the mission harder, but nearly endangered both the PJ masks and the city.
