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?

While visiting a golf course on the island made by the residents, Igarashi witnesses Tonbo's incredible super shot. Intrigued by each other's golf skills, they decide to play together again the next day. While playing, Tonbo opens up about her love for the island. Returning at sunset, Gon encourages Igarashi to experience the island's starry sky, under which Tonbo reveals her past.

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.

Igarashi heads to Kagoshima to find a young golfer who can match Tonbo, who refuses to leave the island. Despite visiting several ranges, no one measures up. On the ferry back, Igarashi learns from Yoko that her niece is a junior tournament winner. Excited, he arranges her visit to Hino Island. Days later, a pickup truck arrives by ferry, with the girl sleeping in the back.

Despite meeting for the first time, Tonbo and Tsubura hit it off and decided to play a round on the island's golf course. Tsubura is amazed by Tonbo's free-spirited style. However, Tsubura's own approach is methodical and precise, with calculated shots. Inspired by each other's contrasting styles, a competitive spirit grows within Tonbo, determined not to lose. How will their match turn out?

The typhoon hit Hinojima directly. At the development center, Tonbo and Tsubura took refuge. Despite the power outage, Tsubura practiced her swings and shared her feelings. After the typhoon, she gifted her 5-wood to Tonbo. Igarashi then found plans for a hidden hole on the island's golf course and took Tonbo to play it.

Igarashi travels to Akureki to meet Mr. Kuta, the designer of the Hinonoshima golf course. He shares his past with Tonbo, who fears leaving the island. The next morning, Tonbo joins him willingly. Mr. Kuta warmly welcomes them and shows them a special one-hole course he created in the forest by a small pond.

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.