Well, there is a plot....
However, the dev had to reduce and adapt to a H-work what took 24 anime episode to reach it's end. So he really had to squeeze it (or make a VN instead of an action game
).
It's true there some really obvious hole in the story / plot, especially when you check the original work...
Still, seem like the dev plan on doing a DLC or at least adding content (check last ci-en post for info)... just wait and see.
PS : true, the action part is well made and you could re-use it to make a full game with it, adding / building lot of more stages...