In the year 1884, Sean O'Herlihy and his comrade, Micheal Flynn, plant a bomb at the Headquarters of the Irish Special Branch of Scotland Yard. The explosion seriously injures Lord Mowbray, Chief Inspector of the Irish Special Branch, and kills his friend, Inspector Ainsley Richland.
They escape to Ireland, but the Mowbray mobilizes the resources of Scotland Yard. The Irish Republican Brotherhood decide to send Sean and his wife Bridget to Mars to avoid capture. Micheal, being a poacher with no family, is sent into hiding in the hills of Ireland.
The
RMS Umbria, is leaving for Mars. Sean and Bridget have to step lively to evade Inspector Hughes, one of Lord Mowbray's able subordinates, to make their departure. They board the ship with help from Doyle Lonnigan, who is Chief Master at Arms for the
Umbria. Hughes is left to watch numbly as the ship lifts. Hughes is sent to Mars to track him down. Mowbray uses his influence to employ a Royal Navy Corvette to speed Hughes on his way.
The O'Herlihys witness a fight that breaks out in the dining room between their breakfast companion and Liam Gallagher, an informer and illegal whiskey maker. Sean attempts to help, only to be cautioned by Lonnigan not to meddle in their affairs as they are rough men. Later, Lonnigan offers Sean a job as a Seedsman on the Green Decks, from which the air on the ship is produced.
Mowbray commissions Gallagher to kill Sean. Lonnigan finds out. While he can't thwart Mowbray's plan, he strive to give Sean an advantage. The result is that Gallagher set to the brig and Sean is confined to his quarters
Pirates attempt to board the
Umbria. Sean distinguishes himself in the defense of the ship. In the aftermath, the pirates happen upon Gallagher in the brig. The pirates recruit Gallagher as he believes that crossing Mowbray is a death sentence. The pirates escape to be chased by the Corvette carrying Hughes. The chase ruins the pirate ship's engine, causing it to crash into Demos. Gallagher escapes into the Free City of Demos.
Sean is the most qualified sailor in the crew to survive from the Green Decks. He distinguishes himself allowing the ship to reach orbit around Phobos.
In the meantime, Inspector Clark is put on the trail of Flynn. Using informants a, Clarke closes on Flynn only to be killed when he gets too close. Mowbray flies into a rage and, despite his condition, travels to Ireland to carry on his investigation. His temper gets the best of him and he assaults an Irish Constable. The resultant cause Lord Mowbray to suffer a heart attack. His superiors arrange for him to be medically retired and send him to Mars for his retirement. He is warned not to continue his vendetta, a caution he ignores.
Gallagher takes a job running supplies to miners in the Asteroid Belt. The miners attempt to rob him, only to have to tables turned. Gallagher elbows his way in to be a partner in the mining operation.
Meanwhile, Sean's ship docks on Phobos and Hughes negotiates his way to Phobos in time to be there. Word has been sent to the crew of the
Umbria to detain Sean. This order upsets Lonnigan owes Sean. They engineer a distraction that allows Sean to escape. Sean meets his contact in the IRB who spirits them away to the surface of Mars.
Meanwhile on Earth, Flynn is warned by the IRB that the police will not stop chasing him after the killing of Inspector Clarke and offer to send him to Mars as his skills are now needed to protect the infant IRB cells that are forming there.
Weeks later, Lord Mowbray exits the liner that has borne him to Mars, he catches sight of Flynn who was exiting the ship. As he attempts to engage Micheal, his heart fails him again and he must watch as Micheal walks. Mowbray is remanded to a physician's care, vowing to get the killers of his friend if it was the last thing he ever does.
