At school, Melissa tells Zack that the following day was going to be Milo's birthday. Zack asks if there is going to be a party and Melissa doubts it, because the last time Milo had a party it did not end well. Since then the only ones invited to celebrate Milo's birthday were immediate family and Melissa. Zack feels sorry for Milo and feels they throw a surprise party for Milo. Talking to his parents on Melissa's phone, both Mr. and Mrs. Murphy are good with the idea as long as it is not at the house. So they suggest on having the party at the Go-kart park, since they have all safety gear they will need in light of an incident. With everyone in agreement, Melissa and Zack begin sending out invitations to some of the kids in school. Surprisingly everyone invited agrees to come, including Amanda after some sneaking persuasion from Melissa. He convinces her to come by helping plan Milo's surprise party.
The next day, Milo is on roof of his house installing a new asteroid warning system. In order to keep Milo busy, Melissa, pretending she sprained her arm, asks him to pick up and deliver a package for her dad. Milo gladly accepts and heads out on his bike. To make sure he does not show up too early for the party, Melissa calls Elliot that Milo is riding a bike that is unsafe and is need of inspection. With Milo kept busy, Melissa, Zack, Amanda, the Murphys and everyone invited helps get everything set for Milo's surprise party. They also bring along a couple things for everyone's safety, like mattresses, extra crash helmets, fire-retardant netting, radiation screens, and glow in the dark T-short. Pretty soon they had the go-kart park Murphy's Law proof and the party is set. All that was left now was the guest of honor.
By the evening, Milo, after Elliot's inspection, had picked up Melissa's package and arrives at where he was to deliver it; at the Go-Kart park camouflaged as an industrial facility. He cycles in and is surprised by everyone, who all wish him "Happy Birthday". Milo is left speechless and delighted. Soon the party starts off with a race around the go-kart track. Everyone has a lot of fun, and soon it was time for cake. However, Milo's parents tell him that Diogee ate all the cake and ice cream, but Milo does not mind it because he already got his wish; He got a birthday party and nothing went wrong. But as he spoke, balloons that were broken free floated past a plane loaded with dynamite. They distracted the pilot and the dynamite fell overboard. Then two lorries loaded with cake and ice crashed, causing their cargo to fly out towards the go-kart park. All three items collided with one another over the go-kart park and created an cake and ice cream explosion. Then after everyone gets covered in cake and ice cream, Milo declares this the best party he has ever had.
- "Weird Al" Yankovic as Milo Murphy
- Sabrina Carpenter as Melissa Chase
- Mekai Curtis as Zack Underwood
- Chrissie Fit as Amanda Lopez
- Christian Slater as Elliot Decker
- Kate Micucci as Sara Murphy
- Diedrich Bader as Martin Murphy
- Pamela Adlon as Brigette Murphy
- The song during the go-kart race, Mister Go, was originally from the band known as Keep Left.