Tommy and his friends cross the frozen tundra to find help for Angelica after she "hurts" her leg. Chuckie needs to learn to be more like Angelica when she leaves him in charge.
Lil learns what life would be like without Phil as her brother. The babies travel to the other side of the moon to stop Angelica from learning all the secrets of the universe.
When Angelica decides to only tell the truth, she accidentally makes the grownups think a snake is in the yard. The babies journey into Angelica's old diaper bag in search of treasure and discover that she was once a baby like them.
Angelica tries to convince her new cousin Simon that she can be as bad as him while all the families visit a re-created Old West town to celebrate Charlotte's birthday.
Tommy and his friends help Chuckie get ready to be a daddy when Chuckie thinks there's a baby in his tummy. Tommy believes that a small goat is his new baby sibling.
Tommy doesn't want to face the reality that, one day, he'll have to give up his bottle. Tommy learns to be a Finster so he can live with Chuckie after his baby sibling arrives.
With Tommy's new sibling soon to be born, Tommy leads his friends on one last great adventure to find Bigfeet while the grownups go searching for Grandpa Lou who has met up with Bob Brine in the woods.
Tommy and his friends think baby Dil is an alien blob and try to return him home to outer space. Angelica has an identity crisis when she gets bubble gum stuck in her hair.
Chuckie enlists a giant Tommy to help Reptar escape from the evil Princess Angelica. Angelica's new robotic nanny manages to keep her behaving almost too perfectly, which terrifies the babies.
When Spike returns from dog school, the babies need to remind him who he really is. After many attempts to make Dil laugh, Tommy succeeds with a rattle--but will Dil ever let him take a break?
Tommy and his friends think an Earth Day event at the park is actually a celebration of Reptar Day. Tommy leads his friends on a mission to retrieve baby Dil who they think the grownups forgot inside the house.
Angelica and Susie try to escalate the building romance between Chas and Kira so Kimi will be over more often. Susie learns the true meaning of what it means to be a flamingo.
Susie's favorite cousin, Edwin, comes to visit, but he's gotten too old to understand how to talk "baby." The babies create a scavenger hunt in the park to attempt to tire out Lou and his friends.