Didi Pickles is pregnant with her second baby, which makes her son
Tommy worry how that will change the family dynamic. The birth comes unexpectedly early, and Didi gives birth to a boy named
Dil. Upon bringing him home, she and her husband
Stu struggle to cope with Dil's constant outbursts. Tommy finds Dil hard to get along with, but Stu assures him that one day, he will be happy to have Dil in the family, and Tommy accepts his newfound responsibility as an older brother. With Dil still causing problems at the Pickles' home, the twins,
Phil and Lil suggest using the
Reptar Wagon Stu has built for a toy contest in
Japan to take him back to the hospital. As Tommy and his friend,
Chuckie argue with Phil and Lil,
Angelica, Tommy's cousin walks in, telling the babies to be quiet. In the process, Dil snatches her Cynthia doll from her. She fights Dil to get the doll back and accidentally kicks the Reptar wagon, which begins to drive away with the babies on board. They speed recklessly through the streets and land in the back of a van, which crashes in the woods. Angelica shows no concern until she finds out that the babies have taken her Cynthia doll, which prompts her to take the family dog,
Spike, to find them and retrieve Cynthia. Tommy leads the babies toward a ranger's cabin, believing it to be the home of a magic "lizard" (a mispronunciation of wizard) who can grant their wish to go home. On the way, they encounter monkeys who hijacked a crashed
circus train. They kidnap Dil, and Tommy's friends refuse to help rescue him, believing they are better off without him, so Tommy sets off after Dil alone. Meanwhile, the family discovers the babies are missing and sets out to find them in the face of the media sensation that has suddenly generated around their children's disappearance. Drew and Charlotte arrive and Drew learns from reporter Rex Pester that his brother lost Angelica, causing Drew to attack Stu. Tommy eventually finds Dil during a storm, but as he struggles to take care of him, Dil continues acting selfishly. Tommy loses his temper and furiously prepares to pour Mash Banana baby food onto Dil for the monkeys to take him again, but his rage and the storm scares Dil into ending his behavior and his tears cause Tommy to calm down, and the brothers begin bonding. After the storm passes, they reunite with Phil, Lil, and Chuckie, who, upon having a change of heart, stop the monkeys from trying to take away Tommy and Dil. Angelica recovers her Cynthia doll and reunites with the babies. As they start to cross a damaged bridge, Angelica falls out of the Reptar wagon and hangs through a gap in the bridge above a raging river. They are confronted by the monkeys, only to be scared off by a humongous wolf named Scar Snout, who attempts to attack the babies until Spike intervenes and fights the wolf, resulting in them both falling off the bridge. Stu, looking for the babies in a
pterodactyl-like glider, sees them from above and crash lands into the ranger's cabin. Believing he is the "lizard", the babies ask him to bring Spike back instead of going home. Stu falls through the bridge and reveals Spike, who survived the fall by landing in the struts of the bridge. The children are reunited with their parents and return home. ==Voice cast==