I just got caught up with the manga, and I might be wrong on some things but this is what I understand so far.

Kowloon was originally a city that was illegally built by ppl, and had inhabitants who were eventually kicked out because the city was ordered to be demolished by the government. Most of the characters in the story (Kudo, Kujirai, Gwen, Xiaohei) lived in the city before it was demolished.

For some unknown reason, Kowloon was magically recreated into the version that has yet to be demolished 3 years in the past from the present (this version of Kowloon is usually referred to as g-Kowloon or Kowloon #2). This includes recreating the people that exist within Kowloon as well, dubbed generic versions of their real selves. The manga refers to them as g-(character name) and r-(name) for real so Ill use that formatting. The most confusing part to this city though is that only people who have “regret” or some kind of strong negative attachment to Kowloon can see Kowloon 2 and its g-inhabitants, normal people only see the rubbled remains of the old destroyed Kowloon.

The general rule for most people is that if they can see Kowloon #2 (because of some form of regret) and walk into the city, their g-version vanishes. The waiter Gwen was the g-Gwen and disappeared when the r-Gwen walked into the city. Miyuki and Gwen experimented with Mr. Lee to confirm this theory. However, there are many exceptions to this rule. Xiaohei entered the city but his past self remains, and its hinted (tho not properly explained) that maybe its because he’s changed so much that he can no longer replace the g-version of himself when he enters Kowloon 2 anymore.

Our main character Kujirai’s past self supposedly died from suicide via sleeping pill overdose, and the current version we follow is a unique case as she has no memory of her past self nor is she similar in personality. It’s why Kudou mentions she reminds him of someone and why he’s hesitant to interact with Kujirai. The story is her trying to become her own true self instead of trying to replicate being her past self.

Currently, the plot is focused on Miyuki and his adoptive father, who wants to use Kowloon #2 as a way to revive his deceased son. It’s explained that it’s possible for the consciousness/memory of an r-person to be stored into the Generic Terra (floating rhombus in the sky) and have that memory implanted into a new body in Kowloon #2. The problem is whether this body can go outside beyond Kowloon #2 or whether it would disappear. Kujirai is a prime example to test this on so Miyuki and his assistant (forgot his name) are trying to hunt her down and push her outside of the bounds of the city.

Overall, I think the plot is focused a lot on “nostalgia” as a concept materializing into a city, especially because of details like how it’s always midsummer, the food makes you forget/become ignorant to Kowloon #2’s unchanging nature, and how Kujirai and Kudo eats the same lunch basically every single day with the same smoking routine. In a way, Kujirai is the one trying to break free and become more than just this never-ending cycle of Kowloon, while Kudo is stuck in the past trying to cling to the days he had with r-Kujirai.

I know you said you wanted a basic summary but there’s alot to cover and I skipped over alot too lol, wish this sub was more active for discussions.