Before the Titanic sails out, Jim installs the electric wires on the ship. When Lord Pirrie and Thomas Andrews arrive on the scene, Pirrie wants the wiring to be quickly finished, while Jim prefers to have backups if one system fails. Pirrie seems to trust the Titanic, saying nothing will fail. He leaves to look for Bruce Ismay, leaving Andrews with Jim. The engineer explains he, as a catholic, has no future in Belfast and would like to join the Titanic to New York, but he has a family which he doesn't want to leave behind, and it would be hard to start a new life once he arrived. He also says he needs a better team of catholic workers.
Lord Pirrie asks Andrews to give Jim empowerment of the completion of the electrics, because they need them on the sea trials. Andrews returns to Jim and promises him the ability to recruit anyone, free Third Class Titanic tickets for him and his family, and enough savings to start a new life in New York. His family, however, will get separated based on gender, it's too late for a cabin for all of them. Jim reluctantly accepts it, formally thanking Andrews.
Once he has the tickets, Jim presents them to his wife. Mary is also reluctant to travel in steerage, but she accepts, convinced Jim will find work in New York.
On boarding day, Jim walks through the ship while his wife and kids wait outside. He finds the cabin which was supposed to be for him and his sons. He takes away the paper name sign, locks the cabin door and takes the key. He then gets his family and wants to return to the cabin, only to discover steward Hart already found the locked door. Jim lets the Sandrini brothers pass and then goes to create a destraction for Hart. He smashes some cutlery on the floor and Hart goes to check on it, leaving the locked cabin unguarded.
Jim opens the cabin door with the key, sneaks his family in and when Hart returns with Jim's cabin mates, the room is already occupied. Hart asks for the Maloney's tickets, but Jim stalls it so long that the cabin mates leave. Jim managed to get his family in one cabin. 
That evening, Jim is having a conversation with someone called Peter Lubov. Mary suddenly drops her cork, which rolls to Peter. He brings it back to her, making her nervous. She reluctantly thanks him, without looking at him anymore. Jim noticed the weird behaviour of her wife.
That Sunday, the entire Maloney family attends a Catholic church meeting in steerage. After the end, Peter Lubov says he's going for "another dose of religion": an Anglican church meeting in First Class. Jim, unlike Lubov, isn't very curious about First Class: he knows every corner as the one who wired the ship. Mary also decides to go. Jim takes care of the children while their mother is off.
In the evening, Mary goes to deck on herself. When Jim finds her again, she says she needed some fresh air, the cabin was too full. 
When Jim returns to the deck where he last saw Mary, he finds her making out with Peter. He rushes to them and starts to beat him up. Mary stops her husband and Peter leaves, refusing to fight back. Mary blames this on Jim for getting them in steerage with horny drunk men.
The family goes to sleep, but Jim is soon woken by Mary. She heard a strange noise, which Jim quickly identifies as steam. Jim gets up to take a look, while Mary puts the kids back to bed.
The Maloneys collect in a large hall with other Third Class passengers. The gate to the lifeboats is locked, with four stewards holding the passengers down, saying there's no danger. Finally, steward Hart takes a group of women and children through the gate, but steward Turnbull quickly cuts off the line. Jim wants to get his wife and children through, but Turnbull is getting bossy. When Peter Lubov intermingles, he grabs one of the stewards and Mary escapes with her children. Turnbull closes the gate again, leaving Jim Maloney and Peter Lubov still trapped down.
An officer enters steerage to inspect the sitiation. A fireman also shows up on the other side, and he points to the trapped women and children. Jim and Peter decide to cooperate until they see Mary and the children in a lifeboat before fighting their own issues. Passengers join the fireman's side, and Turnbull opens the gate to drag Sandrini in, unwillingly allowing Jim and Peter to escape.
When he reaches the promenade deck, he sees Mary is holding hands with another man. Before he can interfere, Mary explains he's helping her. Lord Manton takes Sean and proceeds with Mary and the kids to the lifeboat. First Officer Murdoch wants to hold them back, but the mighty Lord Manton overrides his order and forces the officer to let them through. 
But when Murdoch whispers to steward Hart that the lifeboat might break if even more passengers board it, Theresa immediately assumes the lifeboat is going to break and bolts back on board the Titanic. Jim goes to find her while Mary stays with the others. The lifeboat is lowered without Jim & Theresa.
Jim returns to their cabin, but Theresa isn't there. Suddenly, he almost runs into Peter Lubov. Without any sign of hate because of his kiss with Mary or the promise he made in steerage (fight to the death once Mary and the kids are safe), they cooperate to find Theresa and split up.
Jim finally finds Theresa, trapped behind a closed gate and rising water behind her. Jim realizes their chances of survival are gone, he makes her hold on tight as they sit down on a set of stairs.
Titanic founders and Jim dies with his lost daughter in his arms.