Molly and Gil see Clam trying to mail himself to his cousin Sam's house to make it to a "Happy Clam Day" party. However, instead, the Guppies and Mr. Grouper get the idea to invite Sam to the school and have a Happy Clam Day party there. All the while, they learn about the mail system and how it works.
Before the Theme
Molly is talking when she notices a moving mailbox. Multiple times she tries to say, "It's time for Bubble Guppies" until a hand suddenly pops out of the mailbox to hand her a letter. Upon taking it, they both say, "It's time for Bubble Guppies!"
Molly and Gil are on their way to school one bright sunny day when the Little Fish see a moving package and Molly and Gil come back to check it out. They notice it hoping, then comment on how weird it is. The voice inside asks if its the mailman and they find out its really their class pet, Clam! Molly asks why he is in the box, and Clam explains he's trying to mail himself to his cousin, Sam. Gil and Molly point out that you can't mail yourself and he's very concerned, since he wants to greet his cousin for the Holiday.
Molly and Gil head to class and everyone asks about the box, to which Molly reveals their pet to everyone. They explain what Clam was trying to do and Oona asks about Happy Clam Day, to which Clam reveals they hold a party! Soon everyone begins to get inspired to make invitations to invite Sam to a Happy Clam Day. They then put the letter inside an envelope and go to put it inside the mailbow.
However, Mr. Grouper informs them it isn't ready to be sent yet, they still need to put some things on it. So he goes over what one must do when they want to send mail. Such as put it into the envelope, Writing the persons name on it, along with his address, and also a stamp.
After the song, Nonny decides to open a postal service. His first customer, Molly has come to mail a package when Nonny informs her she'll need stamps first, the more it ways the more stamps it'll need. She reveals that the box is filled with pink feathers, so it will be light. Nonny puts it into a machine and it reveals it will need just 2 stamps. And so, Nonny puts on the 2 stamps before Oona brings him her package.
She reveals she's sending a watermelon, which they think will be heavy and put it onto the stamp machine. It turns out to need 9 stamps! And so, upon stamping it Oona leaves.
Molly talks about the many things someone can send in the mail when she sees Gil holding a "letter", a giant B. She tells him and Gil puts down the B before revealing he's scared of B's, which then proceeds to chase him!
In class they're preparing to Mail the invitation and they go over everything. Mr. Grouper then remembers the snap and one of the little fish points out where the stamp goes. Clam is pretty happy, Deema then wants to party and starts to sing!
...Until she's reminded they need to wait for Sam to come first! So they head over to the mailbox, which is some feet behind them and Molly goes to put the letter inside, until Deema gets emotional about it and she lets her say goodbye first, then drop it inside.
Mr. Grouper then talks about the Mail Box, then explains the person that comes to pick up the Mail is a "Mail Carrier", who then drives off in the Mail Truck. And then she takes the mail to the post office, where the mail carrier who works on Sam's street will deliver it to him.
The guppies go outside to play with Gil pretending to be a mail carrier from outer space. Goby is pretending to be the boss as he tells the story. He asks if the mail is sorted and ready to go, then explains he needs to get the Space Post Office before it closes. Gil claims that nothing could keep him from delivering the mail and he seems to do fine. Though begins to face rain, snow, but when another storm occurs it messes up his windshield and he spirals out of control!
By the time he recovers, Gil sees he's right before the space office but notices how messed up the mail got so he needs to hurry up and sort the two colors: Red, and Blue. He then sees he also has to mail packages and quickly sorts them, and the mail into their proper locations.
And with the mail all sorted, Gil launches the mail into the space post office! And so, everybody lived happily every after...
Molly's Mail carrier Segment
Molly thinks that being a Mail Carrier could be pretty exciting when Gil suddenly comes by dressed as a cowboy and he explains that in the past, people used the Pony Express and how dangerous it was. Molly questions this, and Gil explains the problems with this, then claims there is a heard of Stamps. Gil mistaken s what she said and upon realizing the problem he hurries to flee but doesn't get away in time and is stamped!
The Guppies are setting up for the party, with Goby blowing up a balloon and Oona giving Molly and Gil their party hats. Deema turns on the music but Oona tells her they still need to wait until Sam comes. The others swim to the snack table, to which Nonny mentions they have Napkins now and Clam's favorite food. The guppies finds Clam sitting by the window, who is very sad since Sam isn't there yet. Suddenly, they see the mail carrier, who shows them that she has a package for them!
Upon opening it, they see a disco ball inside! And suddenly Sam shows up! Sam is very happy and everyone wishes a Happy Clam Day. Deema then turns on the music again and all the Guppies sing him a special song to celebrate as the episode ends!
Molly and Gil are very happy by how everything turned out and that Sam showed up. Gil comments that he wishes for a Letter when Molly points out the B behind him yet again. Causing Gil to flee in terror as it gives chase and she giggles.