Two bicyclists, one a Londoner, the other an American geologist named Ward, out for a pleasant bicycle trip in the idyllic Thames valley, meet at a small village inn on the west bank of the Thames. Their parlor chat turns to the subjects of mountains and volcanic eruptions, and the difference between small vent-hole eruptions and large fissure eruptions. The Londoner speaks:"Let us be thankful," I said, carelessly, "that such things don't happen in our own times." He eyed me curiously. "Haven't happened, you mean," he answered. "We have no security that they mayn't happen again to-morrow. These fissure-eruptions, though not historically described for us, are common events in geological history — commoner and on a larger scale in America than elsewhere. Still, they have occurred in all lands and at various epochs; there is no reason at all why one shouldn't occur in England at present."Cycling the next morning, stopping on a river bridge, the Londoner hears a frantic cry and sees a man running on the river tow path as though being pursued by a wild animal."I glanced back to see what his pursuer might be; and then, in one second, the whole horror and terror of the catastrophe burst upon me. Its whole horror and terror, I say, but not yet its magnitude. I was aware at first just of a moving red wall, like dull, red-hot molten metal. ... I think I can recollect that my earliest idea was no more than this: "He must run, or the moving wall will overtake him!" Next instant, a hot wave seemed to strike my face. It was just like the blast of heat that strikes one in a glasshouse when you stand in front of the boiling and seething glass in the furnace. At about the same point in time, I was aware, I believe, that the dull red wall was really a wall of fire. ... a second wave from behind seemed to rush on and break: it overlaid and outran the first one. This second wave was white, not red — at white heat, I realized. Then, with a burst of recognition, I knew what it all meant. What Ward had spoken of last night — a fissure eruption!"How will he escape? Is Ward toasted? Will the torrent of molten rock follow the river valley to London? Can he warn people in time to flee and save lives? Will London be destroyed? ... !