Kakegurui is one of those series set at a school designed to be its own little world with its own special set of rules. In this case, that world is focused on gambling.
The school is a private academy whose students all come from rich and privileged families - the children of politicians and businessmen. In their world, valuable skills include knowing when to take risks and being able to tell when your opponent is bluffing. The school has a system where you can gain power through gambling - but if you fail, you can also lose power and essentially become everyone else's slave.
Ryota is one of those failures. He ends up owing a lot of money to Mary, another student, which means he's now basically her pet. He's considering transferring to another school when a new student arrives and shakes everything up. Yumeko is something many of the school's students have never faced before: a compulsive gambler who doesn't really care if she wins or loses, as long as she gets to experience the thrill of taking risks.
