In the Spring of 1853, the young, ambitious Edward Lowry is thrilled to become Samuel Colt's London secretary. Colt has built a brand new weapons factory on the banks of the Thames, and is eager to sell his new six-shooting Colt revolvers to the British government. Meanwhile, the weapons factory offers an unmissable opportunity for a group of Irish men, members of secret organisation "the Molly Maguires", who have come to England determined to seek justice for wrongs suffered in their home country. They plot to steal weapons in order to kill the Leader of the House, Lord John Russell, Britain's prime minister at the time of the Great Irish Famine.

Edward meets and swiftly falls for Caroline Knox, a Colt factory worker and the sister-in-law of one of the Molly Maguires. However, when all but one of the Molly Maguires are thrown out of the factory on suspicion of stealing parts, Caroline is coerced into joining their plot and has to distance herself from Edward.

Although he initially encounters resistance from the British government, Colt's plan to expand his business attracts the interest of a shadowy ally, the M.P. Lawrence Street, who is trying to further the career of controversial politician Lord Palmerston. Street engineers a string of illustrious visits to the factory, by the Hungarian politician Lajos Kossuth, Prince Albert and Charles Dickens. But Colt's factory is hampered as his workers are repeatedly attacked by men from a rival weapons factory, at least until Colt's murderous security chief strikes back. Progress is also interrupted when Colt is forced to return to America to solve a problem in his Connecticut factory, leaving his dissolute brother James in charge of his British operation.

While Colt is away, Edward begins to have serious doubts about his chosen career. A visit from his cousin in the army and a drunken incident in a bar remind him of the violence of Colt's product. His love for Caroline, combined with his growing anxieties over the ethics of gun manufacture, makes him secretly hope that Colt's factory in England will fail.  He starts to provide information to members of the British government that will ultimately deal Colt's export business a critical blow.

Matters come to a head after Colt's return from America, when the Molly Maguires attack the factory, hoping to steal weapons. As Colt finally achieves breakthrough sales to the British navy and army in readiness for the Crimean War, Caroline and Edward are drawn deeper into the Irish conspiracy, with tragic results.