The story opens on a Maryland highway, where Dr. Curtis Johnson drives too fast while his wife Louise holds back her hair and laughs. Their destination is a truck farm owned by Hamon Dell, a former biochemist who quit the biological warfare center at Camp Detrick. Dell has been sending fruit and vegetables to former colleagues. The Johnsons discuss whether Dell's pacifism is a futile gesture or something more. This domestic scene—dust, lunch baskets, tomatoes—gives way to a confrontation with gaunt, hermetically sealed survivors from a future where Earth's atmosphere is polluted with virulent aerosols. The catalog subjects list this as a short story about scientists and weapons, but the excerpts show that the weapon is not a stone or spear but a tomato, and the scientists are both perpetrators and targets.

The Scientist as Target and Match

The catalog subject "Scientists -- Fiction" is accurate but incomplete. The story presents scientists not as heroes but as what a future survivor calls "the matches" that must be removed to prevent civilization's destruction. Dr. Johnson is told he is a "critical tributary" in a probability river, and that altering such individuals "snips off branches of knowledge before they come into fruit." The future survivors have already killed Dell. Johnson's own guilt is invoked: "your own sense of guilt tells you that you and Dell and others like you are literally the matches." The story thus inverts the typical SF trope of scientists saving the world; here, their knowledge is the fuse.

Probability as a River, Not a Line

The story's speculative core is a mathematical model of time as a "Prime Continuum" composed of "billions of separate bands of probability running side by side." A survivor named Sark explains that changing a tributary—a key individual—can alter the main stream. This is not time travel as commonly depicted; the survivors have not gone back in person but are "controlling critical tributaries" from their own era. The language is precise: "You may change one of these tributaries, dam it up, turn it aside, let it reach the main stream at a different point." The excerpts do not reveal how this control is achieved, but the theory grounds the plot in a logic of intervention rather than paradox.

The Tomato as a Loaded Object

Tomatoes appear three times in the excerpts: in Louise's lunch basket, as a gift from Dell, and finally as a weapon. Sark "reached to a nearby table and picked up a tomato" in the closing lines. The fruit that symbolized Dell's peaceful farming becomes a tool of threat. This recurrence gives the title "A Stone and a Spear" an ironic twist—the actual weapon is a tomato, a product of the very agriculture Dell turned to. The story's structure moves from a casual drive to a philosophical confrontation, with the tomato bridging the mundane and the apocalyptic. The catalog does not mention this motif, but it is central to the story's texture.

Readers should attend to the story's use of everyday objects—a car, a lunch basket, a tomato—as carriers of thematic weight. The dialogue between Johnson and Sark is dense with mathematical analogy and moral accusation, but the physical details anchor the speculative argument. The excerpts leave unresolved how the survivors' plan works and whether Johnson will be killed or converted. The story rewards attention to its layered structure: a road trip, a debate, and a confrontation that reframes the opening banter about stones and spears.