During the mid-20th century, the dominant view among UFO researchers was that flying saucers were highly advanced hardware piloted by biological entities originating from other star systems. Vallée, a trained astrophysicist and computer scientist, grew increasingly skeptical of this nuts-and-bolts materialist approach.

: Medieval European folklore regarding fairies, elves, and gnomes.

: The legendary cloud-realm mentioned by the 9th-century Archbishop Agobard of Lyons, where sky-ships supposedly sailed.

The central thesis of Passport to Magonia is that modern alien abductions are identical in structure and psychological impact to historical tales of interactions with the supernatural. Vallée brilliantly connects the dots between:

Vallée notes that the "little people" of Celtic lore and modern extraterrestrials share highly specific behavioral tropes. Both are known to paralyze witnesses, take humans to other realms where time behaves differently (missing time), perform invasive medical or breeding procedures, and leave physical trace evidence in fields. Review: Passport to Magonia (1969) by Jacques Vallée