A group of friends are reunited at their high school reunion, and are transported back in time to relive their high school days.

The first half was strong for feeling mysterious for why they had been transported back in time, and also a secondary mystery of who is sending them threatening texts. It's also strong for having adults that very much behave like adults even if they are in teenage bodies, and for the characters all having realistically conflicting motivations.

Unfortunately, the story feels much weaker once the mystery is revealed, and as a result the story evolves into a character drama. I feel like it also plays hard into the Japanese culture surrounding regret and grudges, which doesn't entirely not apply to other cultures, but it perhaps comes off too strong, making certain characters hard to sympathize with, as there is a point where even if other people can empathize, it doesn't mean you should just be forgiven for everything. I think Those Summer Days may have also benefitted simply by digging deeper into the details, as twenty chapters isn't much space to cover everything, and it whizzes by at a pretty swift pace.

If you want to read this because of its obvious similarity to Orange, in my opinion Orange is better, but you'll probably find things to enjoy about Those Summer Days as well.