As they adjust to life in Tokyo, Haruka Nanase and Makoto Tachibana prepare for their university's opening ceremony. However, while running some errands, they find themselves representing Iwatobi in a regional mascot wrestling tournament. Unbeknownst to them, the rest of the Iwatobi and Samezuka swim club members are watching the event's live broadcast at a local festival. While also adapting to the quick change of pace, Ikuya Kirishima and Hiyori Toono discuss what moving from the countryside to the city was like. Crossing paths with old friends on the bustling streets of Tokyo, the curtain rises on a new chapter in the boys' lives. [Written by MAL Rewrite]

Free!: Dive to the Future - Soushun no Build-up! premiered at a special Free! screening event on June 23, 2018. The special was later included with the Blu-ray and DVD release of Free! Dive to the Future on September 26, 2018.