Tommy Baldwin and Joe Dugan are tasked with safely transporting a valuable diamond. If they succeed, they will be greatly rewarded, but if they fail, their fate will be dire. When they go to collect the diamond, a gun battle breaks out, a canister of knock-out gas is released, and the diamond is stolen. The police arrest everyone in the vicinity but are unable to find the diamond. Curiously, all those arrested and released board the steamship Asiatic the next day, traveling from Shanghai to San Francisco. Tommy and Joe only have the voyage to discover who stole the diamond and they suspect the seven people from the previous night. However, as the journey progresses, more bodies appear, and clues are scarce.