Melissa and Zack throw Milo his first surprise birthday party, the only real birthday party he's ever had. 
Upon being invited to Milo's upcoming birthday party and learning that it has for years consisted solely of Milo, his family, and Melissa, Zack questions why they can't arrange a larger scale party for their friend. After talking it over with Brigette and Martin Murphy, the pair agree to hold it at a go-kart track for safety and insurance purposes. They then go about the process of inviting various classmates, with Melissa convincing Amanda to serve as party planner. Melissa also convinces Milo to go pick up a package and then deliver it to the party site, alerting Elliot Decker so that he delays Milo's efforts.
Between the Murphys and the students, the go-kart track is soon prepared, and the party begins once Milo arrives. While Murphy's law does come into effect, various precautions taken prevent anything from going catastrophically wrong. Diogee gets into the cake and ice cream, but Milo is not upset, as he has finally had a real birthday party for the first time in years. An accident then causes an explosion of cake and ice cream that lands on the party, to Milo's delight, before the guests are forced to flee from a small group of ducks.
You can read a full transcript of the episode here.