With Kaori heavily relying on a Chekhov’s gun of a diary to remember Yuuki each week, it was only a matter of time before it went off, i.e. got lost. Losing important things is as much a part of life as having friends, and when you rely on one for the other, you’re living dangerously.
Kaori loses the diary after her first fight with Yuuki, which is entirely his fault. He’s pissed that Kaori’s friendship with Shogo is publically recognized by the class before his. He doesn’t like how Shogo seems to be talking to her behind his back. And he really doesn’t like it when Kaori talks at length about how great Shogo is. Shogo+Shogo+Shogo=a fight, fueled by Yuuki’ selfishness.
His punishment is swift, due to Kaori not only losing the diary but also the sign on the wall reminding her Hase is her friend. The next week, she doesn’t remember him, and acts just as cold as when they first met. But Shogo assures Yuuki what Yuuki already knows: she didn’t throw the diary out intentionally, over one little spat. Yuuki’s initial appeal to Kaori doesn’t go well, so he skips school to look for the diary.
During this time, Kaori gets the feeling she’s forgetting something) really important to her. Urged on by Shogo, she seems to remember enough to end up at the riverbank where she lost the diary, where she finds Yuuki covered in mud and hand cuts, and finally finds it with her there. By then, she already knew what the important thing was that she forgot: Hase-kun.
I really liked this episode, which felt less like forced drama and more an unfolding of what was a very likely scenario—losing the diary—combined with that time-honored friendship milestone of the First Fight. The ending in particular was very touching. I will say it’s probably in Kaori’s best interest to keep a back-up diary, or possibly a private blog. And it would seem Yuuki can rest easy: if Kaori liked Shogo or even considered him a close friend, she’d have forgotten him…right?