Molly is going to have a new sibling. The Guppies get excited about babies; the children go to the hospital to meet a new baby.
The episode starts off when Molly greets the audience. She is interrupted when she heard Gil saying, "Goo-goo ga-ga." She goes up to him and starts laughing. She tries to start the episode again but again was interrupted by Gil, who was pretending to be a baby. He then starts asking for his bottle, still speaking like a baby. He says "ba-ba-ba" until Bubble Puppy gives him the bottle. Molly laughs and says, "It's time for Bubble Guppies!"
Gil and Bubble Puppy were on their way to school when they saw Molly and Mrs Peekytoe. They go and say hi and Mrs Peekytoe says that he must have been so excited for Molly. Gil was a little confused until Molly told him that she was going to be a big sister. At first he said, "Cool," without thinking about it. But he realized what she said and he got so excited and started wondering what the baby looked like. And Gil tells the exciting news to the viewers. During the excitement, he and Bubble Puppy accidentally crash into each other. Gil asks Molly when he can see the baby. Molly explains that her mom went to hospital. Mrs Peekytoe's bus arrives and assures Molly that she is going to be a great big sister. Excited to go tell everyone, Gil and Molly make their way to school.
When they arrived to school, Gil tells everyone that Molly is going to be a big sister to a baby. Deema and Oona get really excited. Mr. Grouper congratulates Molly. Deema asks Molly if she would bring the baby the next day so that they can play with it. Mr. Grouper says that the baby is going to take awhile to play and Molly says that her mom told her that when babies are brand new, they can do very few things. Mr. Grouper and the guppies consider what little babies can do. After, the guppies sing "Hey Baby".
Then, Oona shows the viewers her stuffed baby kitten. And Oona gives her kitten a bottle of milk to drink. Mr. Grouper asked Oona if her kitten had a name and Oona said her kitten's name was Marie. Mr. Grouper said that was a nice name. Gil asked Molly what they would name the baby. She didn't know yet. Deema said that she used to have a turtle named Beyonda, but she doesn't know if she was ever a baby because when she got her, she was already big. Mr. Grouper reminds the guppies that every animal start off like a baby and that baby animals go by different names from the grown up animals. Oona said, for example, like we call baby cats, kittens; Goby added that baby frogs are called tadpoles and Nonny said that baby oysters are called spats. There was a silence and Mr. Grouper admitted that he did not know that. The guppies sing "The Baby Animals Song".
Deema goes and teaches the guppies, acting like babies, to do stuff. She shows them a toothbrush and tells them that they use it to brush their teeth. Goby reminded her that they are babies and they don't have teeth yet. The guppies begin to laugh. Molly asks Mr. Grouper who's going to teach her little brother or sister stuff. He assures her that Molly herself and her parents will do and that a baby can learn things on its own. Oona said how can they learn by itself since it's only a baby. Mr. Grouper said babies can use their senses to learn about the world around them. The guppies sing "Come To Your Senses".
Nonny tells the viewers that Mr. Grouper is going to read a story and invites them to go. Mr. Grouper asks the guppies what story do they want to listen to. Gil asked him if he could make one up. He then asks them what should the story be about and Goby said about a baby and Gil agrees. Mr. Grouper invites them to tell the story. Molly said she couldn't wait to tell stories to her baby brother or sister. She told Mr. Grouper that she loves fairytales. The guppies sing "Once Upon a Time".
Nonny and Goby were playing baby with a doll house. Nonny said that it's time for the baby to go sleep. Molly said that she should go to her bedroom. Just then, someone calls Mr. Grouper. He seemed to be very excited about something. Molly approaches him and Mr. Grouper said that it was for her. Molly takes the phone and it was her father. After she was done talking, Molly told us that she's a big sister and everyone congratulates her. Then she said her daddy was going over to pick her up and take her to the hospital. The guppies say goodbye to Molly. Oona gets sad; she says that she hopes they can see the baby before its all grown up. Nonny tells her that babies take a really long time to grow as big as them. They start talking about that babies do at home, Goby said the baby can eat in the kitchen, and Nonny said that the baby can also sleep in the bedroom, and Oona said that the baby can also snuggle in the living room, while hugging Nonny. Deema says there's no place like home. The guppies sing "Happy To Be Home!".
Gil tells the viewers that when Bubble Puppy was a baby dog, he had to teach him tricks like sit and stay. He shows the viewers how he sits and after, how he stays. While explaining to the viewers how he stays, Bubble Puppy sneaks up behind Gil. And when Gil looks back to where Bubble Puppy was, he wasn't there. He started calling for him, to which he barks, startling Gil, but he laughs. Mr. Grouper comes and says Molly called and said they could go to the hospital and see the new baby.
Everyone goes to the hospital and they see Mrs Peekytoe. She greets then and that she just saw the new baby. "Absolutely adorable," she said. Then they hear and see Molly with a doctor who is taking her to see the new baby. Molly invites her friends and the viewers to see the baby. She almost runs into a shark as she follows the doctor. The guppies go see the nursery's window, they see a lot of babies. Gil asks Molly which one it is, to which she doesn't know. The doctor shows up and tells them to come in. Molly sat in a special chair, with her hands already washed. The doctor introduces Molly her new baby sister. All the guppies said "A baby girl!" The doctor gave the baby to Molly and the baby started giggling at Molly. Molly says hi to her little sister and she starts giggling and waves hi. The rest of the guppies came up and Deema asked Molly what the baby's name was. Molly said that her name is Mia. Mr. Grouper said that was a pretty name. Molly and Mia. Mia continued giggling at Molly and she told her that she's her big sister. Mia continued giggling. Mr. Grouper said that Mia liked the sound of Molly's voice. All the guppies thought that was cute, but, then, Mia started crying. Everyone got a little worried. Then, Molly calmed Mia down in a gentle way, telling her that she was all right. Molly sang a lullaby, "Off to Sleep You Go". Mia fell asleep. Molly tells Mia that she loved her. In the final scene, we see the pictures of all the guppies as babies, finishing the episode.