The game begins with Sly Cooper and the rest of his gang, some unknown to the player, launching an assault on Kaine Island. Sly is here to try and get into the Cooper Vault, which is where all of the treasure that has been found by his ancestors was held.
When they got there, they find a heavily fortified fortress and Sly realizes he's going to need more than just the three of them to get into it. With the help of some newly recruited members, Sly manages to get to the top of the island and to the vault. As he opens it, he is attacked and restrained by Dr. M, the owner of Kaine Island.
As he is being restrained, Sly thinks back to how he got into this situation, including how he managed to recruit the newest members of the Cooper Gang.
