Keiichi, Belldandy and Urd, and the Motor Club visit a lakeside hotel maintained by a girl named Chieko, but she turns out to be a spirit who wants to hold Keiichi to a promise that his grandfather made to her long ago.
Summary
For the motor club's annual "Summer Endurance Training Camp", Keiichi takes the club as well as Belldandy and Urd to the Honda Lodge, an old, lakeside manor supposedly run by a friend of his grandfather's. When they arrive, Tamiya is mad at Keiichi for bringing them to a place without any girls, but Belldandy and Urd immediately sense that something is off with the place. The front door opens, but just when Urd is going to attack with a demon exorcism program, a maid comes out and introduces herself as Chieko Honda, the lodge's temporary housekeeper. However, the moment she looks at Keiichi, she leaps forward and hugs him, happy that he kept his "promise".
Inside, Chieko is confused as to how Keiichi doesn't remember either her or his promise, but Keiichi insists that he has no idea who she is. Annoyed, Chieko pulls out a photograph of her and a young man named Hotaru-No-Suke, whom Keiichi realizes is his grandfather who bore a striking resemblance to him when he was young, but even more surprising is how the photo is dated to August 12, 1930 and that she refers to him as Hotaru-No-Suke.
While the Motor Club is busy singing, Belldandy takes Keiichi aside and tells him that Chieko is a Shinnentai, a spirit of a person who died with a desire unfulfilled so strong that it manifests a physical form. Chieko appears behind them and grabs Keiichi so he can sing with the others, but she actually sedates and locks him in a bedroom so she can possess him. Belldandy and Urd realize what Chieko is trying to do and bypass the door's lock by moving through reflections inside the bedroom. Urd tries to use an exorcism spell on Chieko, but it doesn't work because she is a Shinnentai, not a demon.
Belldandy tells Chieko that Keiichi is not Hotaru-No-Suke, but Chieko says that doesn't matter. As she says, she had known Hotaru-No-Suke only for two days in the summer, but when she was ill, it was his presence and the promise he made that kept her alive before she passed away. However, even after she passed, she swore to wait for him to return, and with Keiichi's arrival, her love finally returned through the generations. Belldandy realizes that Keiichi was drawn here through destiny and asks Chieko what promise Hotaru-No-Suke made to her.
Belldandy wakes Keiichi up the next morning and shows him to a garage with a motorcycle parked inside. Keiichi immediately identifies it as a Brough Superior, the "Rolls-Royce" of motorcycles which his grandfather rode. Chieko says that he can have it if he gets it running and that Hotaru-No-Suke promised to take her on a ride once around the lake. Thus, for the next few days, Keiichi spends the day working out with the motor club while he works on fixing the Brough Superior at night. Urd asks Belldandy if she's alright with Keiichi going along with Chieko's request, and Belldandy says that she understands how Chieko feels—staying alive across generations because of a promise—because it's how she would feel.
When the motorcycle's restoration is finished, Belldandy and Urd see Chieko and Keiichi off as he rides around the lake. As Chieko takes in the wonderful experience, Keiichi tells her that his grandfather left his motorcycle here and never returned because he had heard that she died and couldn't bear to come back, making Chieko realize that Keiichi truly is Hotaru-No-Suke's grandson. Keiichi tells her that the last bend of the ride is coming up and offers to do another lap, but Chieko says that once is enough and kisses Keiichi on the cheek, which was the last part of the promise that she and Hotaru-No-Suke had made. Chieko thanks Keiichi, but by the time he returns, he finds himself the only person riding the motorcycle.
With their time at the lodge over, Keiichi decides to leave the Brough Superior in the garage because technically, his grandfather, not him, had promised to take Chieko on a ride around the lake. Thus, the motorcycle will forever stay here to fulfill that promise. It's a decision that touches Belldandy, who tells herself she was glad to come here.