Dr. Mark Muir, a young American metallurgist, persuades American tycoon JP Morgan to hire him for the biggest shipping project the world has ever seen: RMS Titanic, at the Harland & Wolff shipyard, in Belfast. The shipyard's visionary Chairman takes the young scientist under his wing, introducing him to Belfast's Protestant elite. Mark quickly catches the eye of the charming, albeit spoilt, daughter of a magnate: Kitty Carlton. But he is also intrigued by a very different woman; Sofia Silvestri, an immigrant's daughter. Belfast, seen through Mark's eyes, is a city ridden with divisions of class and religion, from which, as a stranger, Mark should be far removed. But we soon discover that Mark is a man with a past mysteriously connected to the Catholic suburbs of Belfast.

Dr. Mark Muir's real name is Marcas Malone, and he is the son of a Belfast Catholic dockworker, Sean. Sean is an old drunkard, broken by a life of unimaginable harshness. His only remaining pride lies in Mark. Now, Sean is worried that if Mark's identity were to be disclosed, he would lose his job at the Protestant - managed shipyard. But Mark is careful and guarded. He starts finding out things about the steel used that lead him into conflict with chief designer Thomas Andrews. In the meantime, social tension escalates. A prominent figure in the labor movement, Jim Larkin, is organizing the workers in Belfast. While Kitty Carlton seduces Mark with her elegant charms, Sofia gets more involved in her fight for freedom, which she interprets in a personal way, turning down her father's apprentice, Andrea Valle, much to her father's grief.

Mark has put his father up in a small hotel. We learn that another man is looking for Mark, someone that Sean fears. Mark's job has made him closer to Sofia Silvestri. Sofia is charmed by Mark: he seems genuinely interested in her. Sofia's friend Emily warns to be careful. But Sofia has other worries. Her father is encouraging Andrea to fight for her. But Sofia won't be forced. In the meantime, Mark discovers a problem with the steel used to build the ship. But science is put to test by the divides in Belfast. The Protestant magnates, worried by the success of the Union, call in the Army to boycott a peaceful march. The Army sabotages the march in the Catholic area, causing accidents that seem organised by Catholic workers. Mark is there to show his respect for the workers.

Walter's death has changed the whole picture. Malcontent breeds amongst workers, especially at the shipyard. Larkin has been forced to leave the city. Andrea, to Sofia's embarrassment, is staying at her house. The Chairman, Lord Pirrie, is shocked, and fights with his former friend Henry Carlton – Kitty's father – whom he holds responsible. He seeks an understanding with Michael MacCann, one of the leaders of the workers' movement. They are both intelligent and moral men. Mark keeps his mind on his job and finally identifies the problem with the steel: impurities. But the steel complies with regulations. Eventually Thomas Andrews will see his point, and a new relationship will form. But work is not Mark's only occupation: he finally kisses Sofia. As he goes back to his father, he is assaulted by Bernard Doyle. He accuses him of having left behind his fiancée, Bernard's daughter Siobhan, who died at childbirth.

Mark's story is fully revealed. When he lived in Belfast, he had a girlfriend, Siobhan Doyle. She was supposed to follow him but never did. Siobhan died soon after. His father told him she had died of TB. Instead, she had died at childbirth without him ever knowing. Mark realises how much Siobhan suffered, and is heartbroken. She was sent to a convent. To his utter shock, he finds out that the child survived: a baby girl. He tries to find her, but the nuns will not tell him anything. An accident occurs that forces him back to work: RMS Olympic, a ship in the same class as Titanic, has had a serious accident. Mark is asked to study the implications. Mark's new discoveries are now less important that delivering on schedule. Pirrie and Andrews discover that Mark Muir is a Catholic, but guard his secret.

The accident has created a terrible workload. Tensions between classes escalate. Pirrie and Michael finally manage to come to an understanding that appease the workers, but not magnate Charles Stokes. Unfortunately, Conor MacCann, Emily's younger brother, is driven towards more extremist acts after he gets fired. The Fenian movement, a Catholic terrorist group, takes Conor in. Mark suggests building a double hull for the Titanic. But Pirrie is unimpressed. It would mean wasting too much time. To try and forget his many concerns, Mark manages to take Sofia away for a romantic weekend. It is a time when love flourishes, giving both of them the illusion that everything will be fine. Henry Carlton has discovered Mark's real identity. Carlton is furious that his daughter, Kitty, has been in a relationship with a Catholic impostor. Kitty is disowned and she decides to leave Belfast. As soon as Mark comes back he is summoned and fired.

Losing his job is not Mark's only problem. Sofia knows his truth. Mark tells her everything. Even about Siobhan and the baby. She eventually forgives him. It is also time for Mark to reconcile with Sean. Mark's situation seems to get better still when JP. Morgan arrives. When told the reason why his chief metallurgist has been fired, Morgan has him immediately reinstated. Mark starts looking for a viable alternative to the double hull, an alternative he finds in higher bulkheads. General Elections are in sight. Home Rule for Ireland is at stake. The possibility that London putting a stop controlling the island is strongly opposed by the Protestant minority, which fears the power the Catholic majority would get. Emily persuades her brother Michael to stand for Parliament, and Pirrie helps and supports his main counterpart in the shipyard.

The Admiralty has opened an inquiry into the Olympic accident, and Pirrie, Andrews, Morgan and Mark are invited to London. Mark is pleased to see his old friend Kitty, who has become a star. At the inquiry, the First Lord of the Admiralty, Winston Churchill, suggests that the yard could serve several ends in case of military escalation. Joanna Yaeger, Mark's old friend from New York, and Morgan's protégée, is forced into a very dangerous position: a spy. With her unparalleled access she can prove to be extremely useful to the Austro Hungarian and German causes. Back in Belfast, Mark faces a new crisis. Andrea, Sofia's old fiancée, has seen her with him and has told Pietro. Sofia is forced to choose. Sofia stays Mark. Finally he agrees to follow Sofia: they will move to London, together. He will leave Harland & Wolff and all hopes to ever find his own daughter. In a breathtaking ceremony, the Titanic is finally launched.

Titanic's sister ship, the Olympic, is damaged again. Mark can't leave. Sofia also has to postpone her plans. Pietro has been injured at work and she must look after him. Mark and Andrews look for new solutions. The gashes in Olympic suggest that the steel is just too weak. Eventually they have to concede that the steel, combined with the sheer scope, is just not good enough. The double hull would have protected the ship - Perhaps the bulkheads too. For this to become a serious problem, Titanic would have to crash into something very solid. An occurrence that seems remote on the Atlantic Ocean. While Mark and Andrews slowly come to their conclusions, Emily is sentenced to an astoundingly unfair six months in jail. But another terrible event occurs: Conor is shot dead while the Unionists start advocating a new, separate State, Northern Ireland.