|Spoiler Alert: This article or section contains details about plotlines not covered in the anime. Do not proceed unless you wish to be spoiled. |
On Friday, 10th April, 2025, after Kirigaya Kazuto hung up his new school uniform, he heard a loud shout from his sister, Suguha, calling him to come down to the garden. When he got down, Kazuto noticed a large object, covered in a bed sheet, next to his sister. Suguha then tore off the sheet, revealing a Bianchi road bicycle. After Kazuto's onslaught of questions, Suguha revealed that she bought the bike with a huge discount from Rinrindou, a bicycle shop that the two had frequented since kindergarten. While examining the bike, Kazuto noticed that the bike was too big for his sister, who then replied that the bike was a gift for him to celebrate his enrollment.
Two days later, at 10:00 in the morning of Sunday, 12th April, Kazuto prepared for a cycling course. Just as he was about to exit into the main road with his bike, he called out Yui, who appeared in his AR sunglasses, connected to the mounted portable terminal on the bike. Kazuto asked Yui to begin data logging and barely dodged her question after accidentally letting out a remark about her Artificial Intelligence abilities.
After cycling for around twenty minutes, Yui suddenly detected electromagnetic waves of the ANT+ protocol coming from Kazuto's bike. At his request, Yui searched for the device transmitting the waves and visualizes a girl the size of Yui. The mysterious girl announced that 907 days had passed since the last ride and that she is being charged by the bike's internal dynamo. Kazuto then realized that the reason why his bike was so cheap was because the previous owner built a power generator in the bottom bracket of the bike, which caused the bracket to get stuck and the bike to be sold for a much cheaper price. He also realized that the battery was probably charged to the minimum level after the time he spent cycling, which caused the device storing the mysterious girl to start up.
Stopping in a parking lot to gather his thoughts together, Kazuto realized that the previous owner of the bike wouldn't have left such a customized interface behind willingly, therefore he decided to find the previous owner and return the supposedly stolen bike to him. Having Yui pass on his voice messages to the girl, he found out that she was named «Cel» and asked her to display the previous traveling route, this revealing the previous owner's home.
Arriving at the destination, a detached house, Kazuto interacted with the intercom and explained the reason for his visit. A woman in her mid-twenties came out and opened the gate, being taken aback the moment she saw the bike. The woman then explained that the bicycle used to belong to her younger brother, a university student, and explained that her brother was denied from riding bicycles due to a certain event in autumn three years ago that left him stuck in the bed for two years. She also explained that due to his weakened body, her brother had shut in his room playing VR games, refusing to go to university or to go cycling, because of this, the boy's father angrily got rid of the bike, which was supposed to have been a present to him for getting into university. Kazuto then asked the woman whether her brother would like to have his old data logger, the one storing Cel, back.
A little while after the woman sent a message to her brother via her terminal, a ringtone started playing from the terminal. The caller explained how to safely remove the data logger from the bike and Kazuto gave it to the woman to pass it on to her brother. After Kazuto shared his contact details with the woman, in case her brother ever decided to buy a new bicycle and come over to Kawagoe to thank him, Kazuto set back on the cycling road to head back home. While cycling back, Yui revealed that she had communicated with Cel and got information on the road surface from her. Yui then asked whether they would ever get to see Cel again, to which Kazuto answered that he was sure that they would meet her again.