Nao Kanzaki was one of the lucky/unlucky few (depends on how you see it) to be selected to join the Liar Game, the moment she opened the box of 100 million yen. The rules are that she has a competitor who also has 100 million yen. If she manages to steal that, she will own 200 million yen total. On the other hand, if the competitor steals her money, she will have a 100 million yen debt.
For a normal person, this would normally be hard, but for Nao you'd think she is the unluckiest person in the planet to be victim of such a game. This is because the thing about Nao is that she is very honest and she is likely to trust anybody. A game called the Liar Game that entails you to lie is definitely not suited for her.