Thaddeus Daniels, a former Confederate spy arrives in San Francisco in June 1866, with the intention of re-igniting the war. He revives a group of pro-Confederate conspirators (Knights of the Golden Circle) to help him carry out his plan of shelling the city from a ship outfitted with cannon. This will be the signal for the thousands of disgruntled Southern men to take up arms again. However, Daniels is recognized by a former foe, whom the spy subsequently murders. The murder leads to an investigation. Soon the U.S. Army lieutenant assigned to the case begins to slowly uncover the conspiracy, which starts a race to stop the conspirators.