@migh It's a very weird concept to explain from scratch, I'd just read Ch. 1 and 2 for context. but I'll try.
So, MC is the dude who nutted this chapter. He aspires to be like his big bro, who was this olympian champion level of runner. But he sucks at running, a lot. Can't even make high school track team.
The tanned girl here is his big bro's wife, another famous athlete. Her whole aspiration is this new method of training that's basically No Nut November to its extreme. Control your masturbation habits (in this case, only nut every 9 days) and it maximizes testostone output, increasing training efficiency (and before you ask, there is some research IRL supporting this. It is nowhere near proven tho). Her research was laughed out of the committee, so she sought to test it out on someone. Making MC go from suck to national high school levels would be great support. So begins the very odd training.
ofc, during the nut days, things tend to get very fanservicy, as you see here. He's gotten bolder and bolder since the beginning, where he was basically the frail shota stereotype in ch. 1. so progress is made in an odd way. MC has a not so subtle crush on his Sister-in-law and at this point they both know it. She's using it to her advantadge, while attempting to not fall to temptation.