Fruit vendor Niccolo Franchetti is conversing with a policeman when something goes "ping" inside his head and he becomes Decimus Agricola, an ancient Roman legionary who a moment before, as he believes, has been involved in a battle with the Parthians.

The victims are referred to the psychiatric department of the Rockefeller Institute for Medical Research, headed by Bill Jenkens, but their condition baffles the staff.

At Jenkins's request, investigating psychiatrist Pierre Lamarque calls in outside aid—his father-in-law Arthur Lindsley, a professor of biology, and some of his academic colleagues in linguistics.

Anti-Semitic demagogue Hans Rumpel is addressing a rally, when to the consternation of his audience his mind gives way to that of Rabbi Levi ben Eliezer, a Polish Jew from 1784.

H. Perkins, a church congregant, is passing the collection plate among the pews when he is overtaken by the persona of pirate Joshua Hardy, and absconds with the offering.

Lindsley and Lamarque continue to brainstorm as the dementia rapidly spreads into a regional epidemic and a mass exodus of citizens clogs the roads out of New York.

Monahan and the academics finally attempt to escape the city, but when their truck crashes they are forced to take refuge from the Isolinguals in a vandalized drug store.

With the aid of store clerk Mr. Bloom they hold off the attackers until finally relieved by an armored car full of soldiers wearing the newly duplicated helmets.

Afterwards, the army surrounds the redoubt of the National Patriots from which the radiation causing the plague has been emanating; the academics’ colleague Dr. Plotnik had triangulated the source.

Per Lamarque: "Seems [Falk] found a complicated combination of harmonics on a long radio wave that would work this ancestral-memory switch, and he and Slidell figured to disorganize the whole country with it.

Damon Knight, coupling de Camp with Robert A. Heinlein, Lester del Rey, A. E. van Vogt and Theodore Sturgeon, wrote that "[t]he first published story of each was distinctive, brilliant and memorable," and that it was these authors, along with a few others, "who brought the new science fiction into being."

"[3] Science fiction historians Alexei and Cory Panshin call de Camp's maiden effort "lively and learned, but ... also clumsy.