When supervillain Ms. Goo Goo starts changing everyone into babies, it's up to Guppy Girl and Bubble Boy to stop her before they also get turned into babies.
The episode starts off when Molly greets the audience along with Bubble Puppy. She is interrupted when she heard Gil trying to get her baby sister, Mia to say his name. Mia babbles happily. Molly goes up to them with Bubble Puppy and starts laughing. She explains to Gil that Mia is just a baby and that she's just learning how to talk. She tries to start the episode again but again was interrupted by Gil, but this time, he was trying to get Mia to say, "Bubble Puppy". Mia responds with "Bubbuh Puppuh!" as Gil giggles. Molly tries to start the episode one last time, but is once again interrupted by Gil, who, this time, is trying to get Mia to say, "Goo goo ga ga". Big Blue Fish swims up behind Gil and says "Goo goo ga ga!" so loudly, it sends Gil flying. Mia, Molly and Gil all laugh, and Molly and Gil say "It's time for Bubble Guppies!"
In Big Bubble City, a supervillian named Ms. Goo Goo and her sidekick, Bun Bun, were robbing a baby blanket store. The giant truck and the stroller were often seen together. Police officers Oona, Nonny and Zooli arrived, but before they can arrest them, Ms. Goo Goo turned Oona into a baby and escaped. This was later reported on the news by Goby.
Meanwhile, Mayor Molly was working in her office and her baby sister Mia was visiting. Mia was getting sleepy, so Molly gave her Cuddle Fish to help her to sleep. Deema then came by and informed the Mayor that police officers Nonny and Zooli were here to see her.
The two police officers entered the mayor’s office with Zooli holding a crying baby Oona. Zooli handed Oona to Molly, who rocked her, calming her down, as Molly told her she was okay. After the police officers explained to the Mayor what had happened, baby Oona started crying again. Molly was able to calm Oona back down with the Cuddle Fish. After Zooli and Nonny left to catch Ms. Goo Goo, Deema suggested that they should call Guppy Girl, but Molly believes that the police could handle things for now.
Meanwhile, at Ms. Goo Goo's secret headquarters, Ms. Goo Goo and Bun Bun were putting the baby supplies they stolen in a giant baby bottle. They planned to set the bottle on the top of the Big Bubble Building and turn everyone in Big Bubble City into babies. Once they succeed, Ms. Goo Goo would take over the city!
Back at the Mayor's office, Molly and Deema were waiting to hear from Nonny and Zooli when there was another breaking news report by Goby. Ms. Goo Goo was seen at the city's biggest toy store and was "toying" with the police. Molly, realizing they need help, changes into Guppy Girl. She asked Deema to watch the babies for a few minutes.
After Guppy Girl left, Mia and baby Oona started saying, "Buh, buh, buh!" With help from the viewers, Deema found out that "Buh, buh, buh" means bottle and that they were hungry.
Meanwhile, Guppy Girl calls Gil and tells him about Nonny and Zooli getting attacked by Ms. Goo Goo, and Gil changes to Bubble Boy.
At the toy store, Zooli was about to call in some backup, but was interupted by Ms. Goo Goo. She turned Nonny and Zooli into babies and gave them bottles for "lunch". As Ms. Goo Goo escapes, Molly and Gil arrived and dropped Nonny and Zooli off to Deema.
Guppy Girl and Bubble Boy hurried to Ms. Goo Goo and Bun Bun. Gil sorried Molly. Then Molly used water power to make Ms. Goo Goo too higher, then Gil used bubble power to make Bun Bun down low. And Molly’s water power makes Ms. Goo Goo’s stroller down low. The giant bottle and the stroller goes down low. Ms. Goo Goo thanks Bubble Boy. Then a sudden, Ms. Goo Goo turns Bubble Boy into a baby. Molly catches him and she takes him off to Deema, who was reading the babies a story.
When Ms. Goo Goo and Bun Bun tried to crank an engine, Guppy Girl hurried to them by stopping Ms. Goo Goo. Ms. Goo Goo tells Bun Bun I thinks it’s time for the Big Giant Goo Goo Baby. When Bun Bun gets a few minutes, then a giant sudden: a giant truck transforms into the Giant Goo Goo Robot Baby. And everybody ran away!
At the mayor’s office, Deema sees a nearby television and Mia turns into a super baby. The adults think it’s a humming bird, a plane, and a baby.