During a race in his second year, Shinkai ran over a mother rabbit as he was eager to win the goal by passing a rider on their left side. This caused him to fall off his bike, but he quickly got up and won the race. While returning home on the same track, he found the bloodied mother rabbit dead on the road with her orphaned child still by it. The thought of him taking a life simply to achieve his lust for victory traumatized Shinkai, and, wracked with guilt, he took in the orphaned rabbit to keep at Hakone Academy (naming it Usakichi), and became unable to pass anyone on their left. Anytime he tried, he instinctively braked for fear of something jumping out into the rode.
This caused him to reject his position in that year's Inter High, much to the disbelief of his team. Fukutomi Juichi in particular was close to rage at the idea that Shinkai wouldn't race, as he didn't understand what would cause such a skilled rider to avoid the Inter High, and slammed him against the wall near Usakichi's cage. But after Shinkai relayed the story of why he couldn't race at his fullest, Fukutomi relented, finally understanding. However, Fukutomi made Shinkai promise to race the next year, as he was going to create the strongest team, and reserved the number 4 bib for him.
Shinkai continued to race in Hakone Academy's cycling club and encouraged Izumida to become a sprinter. When Toudou Jinpachi and Arakita Yasutomo later heard of Shinkai's troubles, Arakita offered to practice with him, so he could become the best at "passing on the right"; Fukutomi and Toudou joined him in helping Shinkai work through his trauma through practice.
Building Hakone Academy's Cycling Team
F-Group Selection Race
While Shinkai doesn't take part in the Inter High on the first day, remaining with his team, he's asked by Fukutomi at their hotel later that night whether he's ready to perform his role for the upcoming race the next day. Shinkai smirks and replies that he certainly is.
Shinkai arrives with his team to a line of photographers before the race. Toudou is angry at him that he's "standing out too much", but Shinkai seems teasingly apathetic to these claims.
During the race, Hakone Academy and Kyoto Fushimi are the only two teams to reach their aces and ride as a complete team. Midousuji Akira mocks Hakone Academy for being weak because they lost to Sohoku the previous day, who has yet to reassemble. After Izumida Touichirou emotionally and passionately defends Tadokoro and calls out Midousuji for being entirely unsportsmanlike, challenging him to a race for the sprint checkpoint, he then zips his jersey because of Midousuji's intimidating aura; however, Shinkai stops Izumida and gives him a power bar to calm down.
Shinkai introduces himself as Hakone Academy's ace sprinter, wearing bib number 4, and displays his skill by quickly riding ahead for the checkpoint. At first, Midousuji makes it seem like Shinkai has him beat, putting on an act of screaming in disbelief and hanging his head. Just as they approach the spectator area, Midousuji's tune changes once more to his confidence in his victory. He sings some of the lyrics to a Japanese children's song called Furusato - a song with lyrics about chasing rabbits on a mountain. Midousuji heard a rumor that Shinkai has been traumatized after hitting a rabbit while passing a racer on their left, so he isn't able to pass people on their left. Knowing this, he sprints ahead of Shinkai before veering as far right as possible, leaving ample space for Shinkai to pass him on his left and even inviting him to do so.
While Shinkai struggles to overcome his fear, his teammates far behind believe he's capable of getting things done when needed, despite Mizuta Nobuyuki talking like Kyoto Fushimi has won. Eventually, after remembering how his team has supported him, Shinkai breaks through his own trauma, and asks Midousuji if he's heard the rumor of the demon that appears on Hakone Academy's flat roads. As that rumor is over a year old, Midousuji hasn't, but he can feel that something has changed. Shinkai then unleashes his "demon self", where his demeanor reverses entirely from cool and collected to wild and frenzied, leaving a panicked Midousuji with no choice but to race him in earnest. Although Shinkai believes he's going to win, by the slimmest possible margin, Midousuji wins the checkpoint instead at the last second by swinging his bicycle in front of his body using his long limbs while sitting down over the rear wheel. Because of this, the sensor attached to his bike crosses the line and is detected first.
This, apparently, crushes Shinkai, and he rides silently back to his team with his head hanging down. Thanks to Shinkai's defeat, Hakone Academy splits apart, Toudou with Fukutomi, Izumida with Manami, and Arakita with Shinkai. Kyoto Fushimi takes this time to reassemble, only to drop their sprinters; Toudou and Fukutomi leave Shinkai and the others to chase Kyoto Fushimi and keep them in check. Before they leave, Fukutomi says Hakone Academy doesn't need any "baggage". This distresses Izumida, who calls out to Shinkai behind him, but after Arakita's subsequent complaining, Shinkai calmly thanks Arakita for helping him recover. Kyoto Fushimi heads for the goal line after taking the climber checkpoint. Hakone Academy, however, regroups (As Fukutomi meant that their team has no "baggage".) and meets with Kyoto Fushimi and a newly regrouped Sohoku near the finish line despite Midousuji's expectations.
As Midousuji and Ishigaki Koutarou break from their teams for the goal, along with Kinjou Shingo and Imaizumi Shunsuke, Fukutomi surprises everyone by calling not for Arakita, his ace assist, but Shinkai, the previously defeated ace sprinter. Midousuji is in shock as he'd defeated him precisely to avoid these circumstances and panics, screaming at Ishigaki to go faster. However, Shinkai reaches them within seconds, confident as ever. As Midousuji tries to put a margin between him and the other aces, launching from Ishigaki, Shinkai launches Fukutomi by pushing him hard from behind. In the end, thanks in part to Shinkai's efforts, Fukutomi wins the second day.
After the Inter High
Shinkai appears very briefly in the background at the end of the film after Izumida bursts out of the sauna at Toudou's family inn.
Sometime during the colder months, Toudou suggests to Shinkai and Arakita that the Hakone Academy third-years should attend a special trip before they graduate. Arakita initially isn't interested, but Toudou convinces him when he says the trip will be to show their gratitude to Fukutomi as their captain. The three of them eventually decide on a theme park they find in a magazine, because one of its mascots, a lion, resembles Fukutomi.
When they arrive at the park, it appears more deserted and lackluster than the magazine portrayed; Arakita complains, and Toudou is worried that Fukutomi is disappointed, but Shinkai assures him that it's simply Fukutomi's default face.
Per Toudou's direction on the "guide rules" of enjoying a theme park, the third-years buy mascot hats from the gift shop, with Fukutomi immediately taking a liking to the lion's hat and Arakita grudgingly wearing animal ears of another mascot. They take several photographs together before hopping aboard a roller-coaster. Arakita acts like it's no big deal, but starts screaming as the ride takes off downhill, while Toudou laughs. While Shinkai and Toudou are unaffected by the roller-coaster, Arakita is shakily impressed, and Fukutomi is frozen stone-faced with fear.
Shinkai grills a meal for everyone afterward and strikes his "Bang!" pose; they take this as a challenge to eat everything he serves. They accept the challenge and begin eating, but Shinkai serves faster than Toudou can chew, so he yells at him, which startles Shinkai so bad that he knocks over a drink onto the grill. The resulting steam burns Fukutomi's face.
In the swan boats at the theme park, Shinkai and Toudou, in one boat together numbered "43", race Fukutomi and Arakita, paired in another boat numbered "12", as if it were a race on the road. Toudou is able to use his "Sleeping Climb" technique to pass Fukutomi. Fukutomi orders Arakita to start pedaling, as like a road race, catching a glimpse of the enemy ahead drives Arakita to chase them down. However, Shinkai switches pedaling with Toudou and unleashes his "demon form" to battle it out with Arakita. In actuality, the boats barely move across the water, and seeing other park-goers watching them embarrasses them.
At the end of the day, the third-years ride the Ferris Wheel to see the illuminations (a light show). Shinkai, Toudou, and Arakita are worried the trip was a disaster that Fukutomi didn't enjoy. Fukutomi suddenly states he has something to tell them. Arakita immediately blames Toudou for the whole thing, which sparks an argument with Toudou that Shinkai can't quell. However, Fukutomi thanks them. To Shinkai, he tells him he did well overcoming his trauma to return as their ace sprinter, the fastest and strongest in history. To Toudou, Fukutomi says there is no better climber who climbs so beautifully and fast, and that the name "Mountain God" suits him perfectly. To Arakita, Fukutomi says his beast-like desire for the goal is admirable, and that he's done well carrying Fukutomi and the team with it. Fukutomi claims he's fulfilled his role as captain thanks to them. Shinkai and the others are embarrassed, but as the fireworks and light show start, a message relayed to them from Fukutomi displays across one of the buildings, which they happily read aloud: "We are strong."
Afterward, they look over the photos taken earlier and are startled to see Onoda Sakamichi and Imaizumi racing on children's bikes in the background of one. Behind them, team Sohoku (minus Makishima Yusuke) rides on a multi-person bike, with Kinjou running behind them, shouting about how Sohoku never gives up.