Igarashi hits rock bottom as he loses his family and his job. He flees society on a ferry to Hinoshima - “Japan’s last hidden paradise.” There, he meets Tonbo, the island's only middleschooler. As Igarashi explores the remote island, he senses something odd—a pervasive golf theme, despite no golf course or known golfers on the island. What secrets does this golf-enveloped, mysterious island hold?

Tonbo impresses seasoned golfer Igarashi with her unique style, playing solely with a 3-iron. Intrigued, Igarashi adapts his wedge with her 3-iron's grip and coaches her on its use. Teaching Tonbo reminds him of instructing his son, Rintaro, evoking regrets about robbing Rintaro of golf's joy.

As Igarashi adjusts to island life, he observes the interactions between Bunpei and Yoko. Golfing daily with Tonbo, he sees how the island has influenced her impressive shots. Inspired by her uninhibited style, Igarashi wants her to try tournament golf. However, he feels there's something unusual about her play. Unexpectedly, a squall hits them on the course.

Igarashi's unease with Tonbo's golf stems from the fact that she doesn’t feel the pressure that comes with a competition. On the island's cliff-top course, he sees his past failures with his son in her. Yet, Tonbo's actions make him reconsider, accepting her as she is. Learning from Yoko that Tonbo hates the idea of leaving, Igarashi realizes that a change in perspective, not location, is needed.

The single-hole course designed by Mr. Kuta on Warizumajima Island, with its beautiful scenery, was a challenging monster hole. Igarashi struggles to conquer it, while Tonbo impresses with creative shots, though Mr. Kuta has doubts about her use of a 3-iron for putting.

Mr. Kuta asserts, "Tonbo's golf style won't work in competition," and takes Tonbo and Igarashi to a challenging practice green. As Tonbo tackles the monstrous green all day, she starts to appreciate the kindness of Igarashi and Mr. Kuta.

The children of the island had to write an essay titled "The Last Summer Vacation" at the end of their final middle school summer. After returning from Akurekijima, Tonbo expressed feelings for the island and new resolutions in essay. The islanders were surprised, and Igaiga worked hard to support her. By spring, at the graduation ceremony, Tonbo had grown taller, and hair reached her shoulders.

Tonbo decided to leave the island, which greatly impacted the residents of the surrounding islands. Amidst the mixed emotions, a farewell golf tournament was held, with just one day left until departure.

Having decided to leave Hinoshima, Tonbo boards a ferry with Igarashi, bound for Kagoshima. On the ship, the two reflect on their time living on the island. Through playing golf with "Igaiga", Tonbo's world begins to broaden. As she starts to look toward the future, the warm presence of the island's residents, who have been watching over her, stays by her side.