Brilliant engineer Mack McAllan, proposes the construction of an underground tunnel linking the United States to England. The colossal effort is approved, and construction begins. Ah, but it's not so easy. The tunnel exacts a heavy toll on Mack's marriage. Also there are many worker casualties as a mysterious underground gas causes paralysis and blindness. Mack's wife, Ruth, becomes a victim of this gas while serving as a nurse in the tunnel, without Mack's knowledge. Also, there are threats to an end of funding to complete the tunnel, and there's a volcano in the tunnel's path. Also, Mack is forced to make a life or death decision regarding the workers. Can this brave effort in construction be successfully completed?