"Grand Blue Dreaming" centers on Iori Kitahara, a university freshman who moves to the coastal town of Izu to attend college. Expecting a fresh start and a vibrant campus life, Iori moves into his uncle’s diving shop, Grand Blue, where he reunites with his cousin, Shinji Tokita. However, Iori’s expectations are quickly upended when he discovers that Grand Blue doubles as a rowdy hangout for the local diving club, whose members are more interested in drinking and partying than actual diving. Despite his initial reluctance, Iori is drawn into their chaotic lifestyle, often finding himself coerced into their antics.

The diving club, led by the eccentric upperclassmen Ryuujirou Kotobuki and Aina Yoshiwara, becomes a central part of Iori’s university experience. The club’s members are a mix of quirky personalities, including the stoic yet kind-hearted Shinji, the sharp-tongued Chisa Kotegawa, and the perpetually drunk duo of Ryuujirou and Aina. While Iori initially struggles to balance his academic life with the club’s shenanigans, he gradually forms close bonds with its members, even as they continually drag him into absurd and often embarrassing situations.

The narrative frequently shifts between the club’s comedic misadventures and their occasional attempts to engage in actual diving. Despite their lack of seriousness, the diving club occasionally showcases genuine passion for the sport, particularly through Chisa, who is a skilled diver and serves as a foil to the group’s otherwise carefree attitude. The series also explores Iori’s relationships outside the club, including his interactions with his childhood friend, Nanaka Kotegawa, and his growing connection with Chisa, which adds a layer of romantic tension to the story.

Throughout the series, the humor is driven by over-the-top situations, slapstick comedy, and the characters’ exaggerated reactions, often involving alcohol-fueled escapades and misunderstandings. Despite its comedic focus, "Grand Blue Dreaming" occasionally delves into themes of friendship, personal growth, and the importance of finding one’s place in a new environment. The story maintains a lighthearted tone, with the diving club’s antics serving as a backdrop for Iori’s journey of self-discovery and his gradual acceptance of the unpredictable life he has stumbled into.