The episode begins with Haruno Takaragi revealing her secret base beneath the lighthouse, where Yu Akeuchi lives. Haruno explains that her grandfather was the lighthouse keeper and she used to play there as a child. She offers Yu the key and suggests making it their new secret hideout. Umika Konohoshi, Yu, and Haruno agree to use the underground chamber as their new operations base.

The trio then holds a strategy meeting for the upcoming duel against Matataki Raimon, a mechanically skilled but reclusive classmate. They decide to gather materials the next day. Umika, Haruno, and Yu meet up and purchase various supplies, including plastic bottles and duct tape. They also enjoy some leisure time, taking photo stickers and trying bubble tea.

The day of the duel arrives, and the mood is tense. The competition is simple: Each team gets two chances to launch a plastic bottle rocket, and the team whose rocket flies the farthest wins. Umika's team, calling themselves "Team Shooting Star," goes first and manages to launch their rocket 103 meters into the air. Then, Matataki reveals his rocket, powered by gunpowder. Their rocket reaches a height of 127 meters and wins the duel.

Despite the loss, Umika takes the opportunity to talk to Matataki. Contrary to Matataki's expectations, Umika doesn't ask for help with the rocket but instead compliments Matataki's glasses and expresses a desire to see them at school again. Matataki is taken aback but doesn't respond immediately.

The episode ends with Matataki showing up at school, explaining that they're only there to avoid failing due to absenteeism. However, they agree to help Umika, Yu, and Haruno with their rocket project until they've accumulated enough attendance days.