Engineers Richard McAllan and Frederick Robinson manage to get financial backing for a gigantic project to build a tunnel from England to America. His biggest supporter is Varlia Lloyd, daughter of one of the backers, and she uses her influence more than once to keep the project going. Mack's wife Ruth is also supportive, although his constantly being away on the tunnel project strains their marriage, and affects his relationship with their son. After years of financial skulduggery and physical obstacles under the ocean floor, the tunnel proceeds as Mack's marriage and his friendship with Robbie deteriorate.
