I didn't understand a single thing that happened in this story.
It's the kind of incredibly detailed worldbuilding that's so intricate the author just...forgot to let the readers in on the plot, I guess? There are about a billion different threads going on, with none of them explained in any real detail.
The basic outline is that in this world, vampires and humans coexist by creating a "blood market" where humans are judged based on the quality of their blood, which they sell to vampires as carefully regulated and categorized food. (Are only vampires wealthy, then? Where does that money come from? Are only poor humans selling themselves? Where do the wealthy humans live?)
Sotetsu is an employee of the Blood Control Bureau, which keeps the blood market regulated and punishes vampires that attack and feed on humans outside of the official establishments. The employees there are mostly "D" grade blood, which means "discard," because it's so unpalatable no vampire wants to feed on it.
Except Sotetsu is the son of the mythical "Ambrosia," a woman whose blood was so pure and powerful that it brought enlightenment to vampires. I guess? But then she died? And for some reason her son's blood is basically poison...from birth, or just from grief? Who knows.
Meanwhile, Mozu shows up, a "gourmand" vampire who's intrigued by Sotetsu's blood but joins the Blood Control Bureau to search for Ambrosia, whom he believes still exists in some form. If not Sotetsu's dead mother, then someone else who has the same qualities in their blood. Mozu moves in with Sotetsu, and it seems like we're going to get some sort of bickering housemates dynamic.
But then we have this whole sideplot on their first mission together, where Mozu kidnaps the person being called "Ambrosia," then goes oh no nevermind I won't ditch the Bureau that fast because it was fake spiced-up blood. And instead of dealing with ANY of this, a variety of other frenetic events happen, we get a new "Farmer" villain introduced, who's feeding the Seed of Life to various humans to try to turn them into Ambrosia I guess? And then Sotetsu saves Mozu's life, so Mozu saves him right back by shoving a seed into his chest, which sprouts and I guess transforms him and grows his arm back? And then Mozu turns into some sort of god? (Who, quite frankly, is very ugly, and Mozu's character design was the only thing I was liking in this story.)
So I honestly just. Have no idea. I'd sort of like to know where the story goes from here, but I also don't think I want to put in the effort to get there. So very little of this made any sense at all, and I'm shocked the ratings are as high as they are, because did I miss something? The dialogue is also messy and nonsensical in a lot of spots, so unless that's a Seven Seas translation issue, there's just...not a lot that's appealing in this story.
I think I'm giving up on the vampire genre in BL. So far every single one has been muddily confusing and ultimately pretty disappointing. I don't get why that is, but here we are yet again.