Ukai Rei (aka Zero, which is Rei in Japanese) saves three troubled teens from a Suicide Website, and together they choose to work as modern-day 'Robin Hoods' of society. However, Rei's friends are caught by an underworld gang whose heist the group had foiled. In order to rescue them, Rei is forced to enter a gambling competition hosted by a rich, rotten old man who is looking for a talented gambler to be his rep player in an upcoming gambling tournament for millionaires from all over the world. The reward money? 100 BILLION YEN.
Note - This story is directly continued in its sequel.