A young teen boy in Leavittsburg, Ohio grew up mean. He ran away from his alcoholic father at the age of fourteen. An old man in Texas takes an interest in him, helping the runaway get a new identity. The boy becomes John Harding.

Harding joins the Marine Corps with his new identity, looking for a chance to get an education and start over. While in special ops overseas, he's recruited by a CIA agent who sees something in Harding besides his knack for languages and combat skill. Denny Strobert knows he's found a killer.

HARDCASE begins in a back alley warehouse in East Oakland, CA, where Harding fights for money, and to keep his skills honed, coupled with the fact he likes it. Harding does odd jobs with the only guy he trusts implicitly, his handler and manager, Tommy Sands. They bodyguard, escort tourists around the Bay Area, and do bail bond, skip trace work for a lawyer named Tess Connagher.

Two events throw Harding's seamy life in the shadows out into the light. Strobert is ordered to enlist Tess Connagher's law firm to act as an intermediary with Harding in order to protect an outspoken Afghani woman whose family Harding protected long ago. The woman's father requested him, and the state department realizes they'll be off the hook if anything happens to Samira Karim while under Harding's care. Secondly, Harding fights a particularly brutal fight with a Russian mob backed fighter named Van Rankin who hates Harding. They'd had words.

The YouTube video of Harding's subsequent beat-down of Rankin goes viral along with many of his prior fights. When Rankin's Russian mobster backer, Alexi Fiialkov pushes for a rematch on the UFC circuit, CIA Agent Strobert sees infinite possibilities since the United Arab Emirates has part ownership in the UFC. Some matches would be taking place in Dubai, where the CIA knows targets of opportunity show up from time to time.

Tess Connagher, both fascinated and repulsed by Harding, cannot stay away from him. She rapidly finds out Harding is not some East Oakland leg-breaker, and being around him can be very dangerous, whether you're his friend or his enemy. She makes a couple of serious errors in judgment - Tess thinks she can control Harding, and she uses him to help Lora, her older sister.