Miles and Loretta are getting ready for Building Day where all the Cosmic Explorers get together to build things and earn the Building Day Holo-Badge. Stella informs the family that they are approaching the planet Conmentum where the Building Day Challenge will take place. When they land on the planet, Miles and Loretta meet up with Mrs. Kitumba and M.E.R.C., Leo and Phoebe leave for the finish line and meet with the other parents. Once inside the Cosmic Explorer Challenge Center, Miles and Loretta meet up with Mirandos, Haruna, and Blodger. Mrs. Kitumba reminds the troop that the point of Building Day is to work together and not to compete with each other. The challenge for the Cosmic Explorers is to build their own vehicles and travel through a course on the planet's icy terrain to the finish line. The troops start building their vehicles. Just as Miles and Haruna attempt to add ice axes and a melting devices to their vehicles, they are told by Mrs. Kitumba that they are to cross the course not to destroy the it. Miles and Haruna both decide to add legs and springs to their vehicles. Loretta adds treads to her vehicle, while Mirandos adds parts to make hers hover.
Once their vehicles are finished they put them into the printer and they go to the starting line with Miles and Haruna talking competitively to each other. Mrs. Kitumba reminds the Cosmic Explorers they will only get their holo-badges by crossing the finish line together and to follow the trail. When they start Miles and Haruna start racing against each other, while Loretta, Mirandos, and Blodger have trouble with their vehicles. When they reach a pile of rocks one of the legs of Harruna's vehicle gets caught and uses a super high jump, but this causes one the legs of Miles' vehicle to lose control and go off course. Harlan, Loretta, Mirandos and Blodger offer to help him and follow him to the top of an ice mountain, but the spinning mechanism of Blodger's vehicle causes the ice to crack and they fall into a hole in the mountain causing all the vehicles to fall apart. Using her Calibandroid Mirandos discovers that the cavern they are in is the mouth of a Cryogeyser a volcano with ice and it is about to erupt. They come up with a plan to escape by using the parts of their vehicles to make a new one big enough for all of them, break through the ice and fly them out, but only by working together.
Meanwhile at the finish line, the parents are waiting for their kids, but M.E.R.C. points out that they have not shown up yet. Maya, thinking they went off course, takes her BlastJet with Leo, Phoebe and M.E.R.C. joining her, while Prospero and Blodger's parents wait at the finish line in case the kids show up. Inside the Cryogeyser, the water bellow the ice starts bubbling while the Cosmic Explorers are build their new vehicle. Outside the volcano Mrs. Kitumba, Phoebe, Leo and M.E.R.C. are looking the kids, when they heard it rumbling and found the vehicle tracks. When they finished building the new vehicle they call it the Iceberg, then the Cryogeyser begins to erupt they scramble into the vehicle and Miles flies them up to the celling and breaks through it. and meet up with Mrs. Kitumba, Leo, Phoebe and M.E.R.C. When they reach the finish line Mrs. Kituma presents the Cosmic Exploers their Building Day Holo-Badges for meeting and surpassing the challenge. As Phoebe admires the Iceberg, Miles and the other Cosmic Explorers offer to build more for the Tomorrowland Fleet
- Justin Felbinger as Miles Callisto
- Fiona Bishop as Loretta Callisto
- Olivia Munn as Phoebe Callisto
- Tom Kenny as Leo Callisto/Blibby Blopp
- Dee Bradley Baker as M.E.R.C.
- Grey DeLisle as Stella
- Sam Lavagnino as Blodger Blopp
- Issac Brown as Haruna Kitumba
- Ivy Bishop as Mirandos
- Cree Summer as Maya Kitumba
- Mirandos earns her first Holo-Badge in this episode (the Building Day Holo-Badge).