Nobara's life gets complicated by the unwanted attentions of an unscrupulous boy at school who threatens to upend her whole life if she doesn't play by his rules. Things go downhill from there.
Haibuki doesn't take it so well, does he? And just when things were getting comfy again between him and Tomoyo. Personally, I think Kaz (the blackmailing stalker) deserves a bit more official punishment than just getting yanked around by his shirt collar, but this is shojo, after all, and they like to gloss over the practical in favor of the emotional.