Rubi Malone is a "problem-solver": a bounty hunter and general mercenary. In the game's prologue, she is hired to retrieve a briefcase that is hijacked by a gang. She does so, leaving numerous dead gang members in her wake, and delivers the case to a hospital. It turns out to contain a human heart, which a powerful man named William Ackers needs to survive. Rubi delivers the case to Ackers's grateful son, collects her fee, and departs.
One year later, Mr. Ackers approaches Rubi in her Texas hideout and hires her to go to Hong Kong to bring back his son, whom Mr. Ackers says has fallen in with a bad crowd. Rubi flies to Hong Kong and consults with a local friend, Ming, who tells her that Ackers is heading up a powerful drug ring. Rubi, with difficulty, kidnaps the younger Ackers and delivers him to his father in London.
However, the "William Ackers" who hired Rubi turns out to be an imposter and a rival of the real Ackers. His bodyguards decapitate Ackers's son, then stab Rubi and leave her for dead. Rubi recovers with the help of a friend, Milo, and vengefully starts to track down the fake Ackers and his gang.
On a tip from Milo, Rubi does a favor for a shady woman named Kafka, performing a theft of a rare book being shipped to the British Museum. Kafka puts her on the trail of "Ackers," who is really a drug lord named Rupert Pelham. The trail leads Rubi back to Hong Kong and then to London again, where she is captured by Pelham's subordinate, Sorrell, and tortured for information. Rubi manages to overpower her captors and escape, and kills Sorrell, but not before he confesses that Pelham is moving in on the real Ackers that night.
Rubi confronts Pelham at Ackers's mansion, just as William Ackers is about to be killed. Rubi duels and kills Pelham's chief bodyguard, Tarantula, by snapping her neck, then decapitates Pelham. Ackers says that Rubi delivered his son to his death, albeit unknowingly. He cannot bring himself to forgive her, but her actions that night are enough for him to refrain from taking vengeance on her. Rubi accepts this and leaves, pocketing a small stack of cash that Pelham threw at her to try to save himself.
Before the credits roll, there is a close-up of Tarantula, whose hand twitches.