A group of wealthy industrialists gathers at the home of C. Aubrey Smith as Lloyd, a self-made millionaire, who introduces them to Richard Dix "Mack" McAllan, the engineer who spearheaded the ambitious Channel Tunnel project and a Bahamas-Miami tunnel concept. McAllan explains that the Allanite steel he developed, along with a radium drill devised by his friend Leslie Banks as Frederick "Robbie" Robbins, could make an undersea crossing between England and the United States a reality. Though the group is initially skeptical, the backing of Lloyd and his associate Basil Sydney as Mostyn convinces them to buy shares in the venture.
Three years into construction, McAllan becomes a worldwide celebrity, but his work keeps him apart from his devoted wife Ruth, played by Madge Evans, and their young son Geoffrey, portrayed by Jimmy Hanley. When he is called away to New York, he learns that public support for the project is waning. Lloyd urges him to use his fame to galvanize backing, while Lloyd’s alluring daughter Varlia, played by Helen Vinson, quietly cherishes him and adds another layer to the press’s attention. Ruth, feeling increasingly isolated, begins to work in the tunnel as a nurse, where she contracts an unknown gas that blinds her. Afraid that her husband will pity her, she leaves McAllan and their son, a heartbreak that drives him deeper into the work and strains his friendship with Robbins.
Years pass, and the project’s toll—measured in both lives and money—continues to mount. The British prime minister, portrayed by George Arliss, and the American president, played by Walter Huston, watch with hopeful anticipation for the tunnel’s completion, envisioning a symbol of unity and peace. Ruth dwells in the countryside with her now-grown son, Geoffrey, who lobbies Robbins to secure him a job in the tunnel. As the end nears, the team faces a new obstacle: a submarine volcano that will force a dangerous detour. McAllan needs more funds to implement the detour, but Grellier, an arms manufacturer, and Mostyn oppose the move. Earlier manipulations by Grellier and Mostyn had tilted the stock market to seize control of the project, and Lloyd suspects their scheme will intensify if the delay drags on.
Varlia attempts a bold tactic, convincing Mostyn to finance the continued work in exchange for her hand in marriage, a move that reshapes loyalties within the circle. The project presses forward, but Grellier’s resistance complicates matters and threatens the entire venture. Then tragedy strikes: the volcanic gases prove too powerful, snuffing out hundreds of workers, including Geoffrey. Grief and resolve collide as Ruth reappears at the site, seeking closure, and McAllan redoubles his vow to push the project across the line. With three volunteers, he and Robbins man the radium drill, braving brutal heat and perilous conditions, until at last they crack through to the American side, sealing a future that once seemed almost out of reach.
