"#4: The Tokyo U Promise from 15 Years Ago: Diary" is the fourth episode of the Love Hina anime series created and released as part of the 24-episode anime series airing on TV Tokyo from April 19th through to September 27th, 2000.
The series featured character designs designed by Makoto Uno, based on those designed by Ken Akamatsu, and possessed the opening theme song Sakura Saku as well as the closing theme Kimi Sae Ireba.
When the anime was later used as the source for a comic based on the anime, Love Hina Anime Comics, the television series, unaired 25th episode, and the Xmas and Spring specials were all included in the series.
Writing in his diary, Keitaro recounts how the Hinata residents discovered his student ID and confronted him on the lie of him being a Tokyo University student. Apologizing profusely for the lie, he begs the girls to let him stay.
With Naru convincing the other girls to let him stay, on the condition that he pass the coming prep exam, Keitaro becomes subjected to performing numerous chores around the dormitory on top of studying. Gaining Naru as a tutor, Keitaro ends up explaining his reasons for aiming for Tokyo University.
After hiding from Mitsune underneath Naru's Kotatsu, an accidental groping leads to Naru throwing him back to his room and ordering him to board up the hole between their rooms the following day. While attempting to board up the hole, Keitaro accidently views Naru's diary, discovering that she also had a promise to keep by entering Tokyo University. Caught reading her diary, Keitaro is once again pushed back into his room by Naru, who continues to ignore him in the following days.
Wanting to make up for his transgression, Keitaro plans to apologize and thank her for her help after passing the prep exam. However, after grading the exam he loses hope after failing the first three answers and surmises that he had once again failed. In an attempt to sneak back into the dormitory to retrieve his possessions without anyone noticing, he runs into Shinobu who alerts the other residents to a burglar. As Keitaro is chased out of the apartments, Naru notices that it was him but decides to let him go. However, after being convinced by Haruka that his situation was similar to what hers was before she was tutored by Seta, she travels to Tokyo University where she finds Keitaro visiting the campus one last time. After Keitaro makes amends for reading her diary, Naru reveals that she was also entering Tokyo University to fulfill a promise; a revelation that Keitaro interoperates as meaning that she was his Promise Girl.