It's Setsuna's birthday, but it seems that Hazuki has something more important to do than attend his girlfriend's party, instead crying off by feigning illness in service of other ends.
In other words, it's Touma who has won out on this particular occasion, as Hazuki meets her at the airport to escort her back home out of the blue. There is at least something of a method to his madness however - having met Kazusa's mother he now knows her future plans, and is insistent that Kazusa come along for Setsuna's birthday to explain these plans for herself, given that they involve her moving to Europe with her mum and thus breaking her previous promise to stick around with Setsuna forever. Of course, Kazusa is none too keen on this idea - firstly because she's just finished a long flight, secondly because breaking someone's heart on their birthday is a bad idea no matter how you look at it, and thirdly because... well, the ensuing argument makes Kazusa's feelings for Hazuki totally clear, which he somehow has managed to be completely blind to the whole time.
With that out in the open, the second half of the episode takes a journey back to Kazusa and Hazuki's first meeting, as the former is moved away from the musical curriculum to be placed with the "riff-raff" - a move which doesn't impress her one bit as her history of poor attendance and lack of effort continues. Of course, Hazuki is the exact opposite of this kind of half-arsed behaviour and wastes no time in pushing Kazusa into at least filling in the paperwork that she needs to - an effort which irritates her hugely, but one that she eventually warms to as more of Hazuki's personality comes into her view, ultimately leading to where we are now.
Although the normally likable Hazuki loses some major marks in this week's episode by acting like a jerk on several fronts ("Please ruin my girlfriend's birthday in the name of honesty, Kazusa"), this did at least make for perhaps the most emotionally charged moments we've encountered in this series - the row between Hazuki and Kazusa had a decent impact to it and worked well, which makes the needless descent into a flashback that spends half an episode explaining the bleeding obvious all the more frustrated. It was already pretty clear how this pair's relationship had developed to where it now stands, so I'm not sure why we needed gaps that weren't really there filled in, and this ultimately served as nothing but a distraction at a point where I'm far more interested in what's going to happen next in the present. Given his behaviour this week, I wouldn't be surprised if Hazuki breaks up with Setsuna before her birthday is over, then sets fire to her little brother in front of her just to finish the job.