The story opens with the eccentric Professor Lucifer descending upon a remote hermit in a flying machine to prove that science has emptied the universe of meaning. Upon crashing into St. Paul's Cathedral in London, the professor and his monk companion encounter a devout Highlander named Evan MacIan. MacIan has just smashed a secularist shop window because its owner, Turnbull, printed blasphemous claims regarding the Virgin Mary. MacIan immediately challenges Turnbull to a duel to the death.
What follows is an erratic, relentless chase across the English countryside, orchestrated by the authorities who are determined to prevent the bloodshed. As MacIan and Turnbull flee from police, magistrates, and self-appointed philosophers, they are continually forced into close quarters. Despite their profound ideological differences—one a mystical Catholic, the other a fiery atheist—they develop a strange, grudging brotherhood under the shadow of their impending combat.
Throughout their journey, they encounter various modern dogmatists and social theorists who attempt to pacify them with hollow maxims of love and progress. Both men reject these modern dilutions, realizing that their fierce disagreement is one of the few authentic things left in a rationalized world. Their flight eventually leads them to a remote island and finally back to a small island community where their duel becomes inevitable.
As the rising tide threatens to swallow their makeshift arena, the two antagonists cross their swords once more. The physical barriers of their ideological war dissolve in the face of nature and shared humanity, leaving them not as conquered enemies, but as brothers bound by an unyielding quest for truth.
