In an unspecified future, a meeting is convened at the house of Lloyd, a wheelchair-bound millionaire, with representatives of the steel, airline, oil and armaments industries in attendance. Having earlier overseen the construction of the Channel Tunnel in 1940, engineer Richard 'Mac' McAllan now proposes to the assembled millionaires a project to construct a transatlantic tunnel between Britain and America. What renders this project possible, he argues, is Allanite, a non-porous steel he has invented, and a new radium drill designed by his friend, Frederick 'Robbie' Robbins. Lloyd argues that the tunnel will help to secure world peace, and after some debate the group agrees to fund the project. The arms manufacturer, Grellier, jokes that the tunnel's completion will enable him to sell weapons to those who would wish to destroy it.

Three years pass and work on the tunnel has commenced from both sides of the Atlantic, with Mac overseeing the British excavation. A worldwide television broadcast reports on the invaluable work of the radium drill and how Mac is an inspiration to the workforce.

But Mac is increasingly putting the tunnel ahead of his home life. For the birthday of Geoffrey, his young son, Robbie has to buy a present on Mac's behalf. Following a summons by Lloyd, Mac then travels to New York instead of going home to Geoffrey's birthday party. While in transit, Mac and his wife, Ruth, argue via televisor (a form of video telephone) about his putting work before family.

Lloyd tells Mac that public confidence in the tunnel is waning and he must make more publicity shoots in order to keep the project in the news. Although resistant to the idea, he accedes to Lloyd's request and begins to appear regularly in magazines and newspapers, invariably in the company of Lloyd's daughter, Varlia, who is secretly in love with Mac.

Without Mac's knowledge, Ruth joins the tunnel project as a nurse, but collapses through the effects of Tunnel Sickness, an ailment caused by a gas that attacks the nervous system. As a consequence she loses her sight.

Believing him lost to Varlia, Ruth decides to leave Mac, but asks Robbie not to tell him that she is blind. When Mac arrives home Robbie tells him that Ruth has left and chastises him for neglecting both her and Geoffrey.

Together with Mostyn, another of the millionaires funding the project, Grellier concocts a scheme to wrest control of the tunnel from other shareholders by fabricating a fall in the value of its shares. Through this duplicity the pair eventually gain the controlling block of shares in the syndicate.

More years pass and, following an explosion, Mac informs the syndicate that the tunnel must be diverted as they are approaching a subterranean volcano.

With the financiers unwilling to invest further money to achieve this, Varlia, aware of Mostyn's unrequited love for her, approaches him with the proposal of a relationship in return for Mac receiving the necessary finance. When Mostyn agrees to Varlia's proposal, Grellier confronts him about his investment of the extra money and advises him to change his mind; when he refuses to do so he is shot on Grellier's orders.

Following the earlier explosion, the tunnel's workforce is reluctant to return to work, but Mac leads a small group of workers to repair the damage. Among this latter group is the now adult Geoffrey. With work now commencing on diverting the tunnel around the volcano, Mac also begins to get to know his formerly estranged son. However, when a further explosion threatens work on the entire tunnel, the closure of safety doors leads to the deaths of hundreds of trapped workers, among whom is Geoffrey. Ruth, who has arrived at the tunnel seeking news of Geoffrey, meets Mac after many years apart and he finally discovers that she is blind.

With the American excavation close, Mac leads four volunteers, including Robbie, into the tunnel to finish the excavation. Through searing heat, they man the radium drill and restart tunneling, eventually driving through to the American side.

The completed tunnel is finally opened with joint ceremonies on both sides of the Atlantic, with a reunited Mac and Ruth looking on.