A captain receives a translation of a thousand-year-old manuscript, written by Brother Parvus, telling of the exploits of his lord, Sir Roger de Tourneville, who established an English space empire based on the feudal system. The unexpectedness of the successes of the medieval British against aliens and advanced technology and the further irony of their tenacious independence helped this novel win a Hugo Award.
In c.e. 1345, when confronted by a scouting spaceship of the Wersgorxian Empire bent on conquest, Sir Roger and his men defeat the invaders, claim the spaceship, and take a single hostage, Branithar. Brother Parvus interrogates the alien, a short, blue-skinned, pig-snouted humanoid, learning his language and teaching him Latin. Sir Roger mounts an expedition against France and then the Holy Lands. He and his people board the spaceship and are launched toward the alien outpost by Branithar, who treacherously engages and locks the autopilot computer in an attempt to make them prisoners.
On arrival, the English defeat the three garrisons on the Wersgorxian outpost, partly through guile and brash strategy, partly through relying on difficulties of translating languages during negotiations, and most overtly by using the aliens own weapons against them. The aliens have become accustomed to terrorizing less technologically advanced species and therefore are unable to handle the brash English; they are overmatched in hand-to-hand combat. During this action, the route home is lost, and Sir Roger becomes increasingly estranged from his homesick wife, Lady Catherine, who is being courted by Sir Owain. Realizing that his battles constitute a war against the Wersgorxian Empire and that Earth might be in peril, Sir Roger seeks allies in alien worlds subjugated by his opponents.
While the alliance is forged, Sir Owain enlists Branithar in recovering the route home and persuades Catherine to search for Earth. Sir Owain forces Sir Roger to a parley and tries to take him prisoner. Sir Roger fights, defeats Sir Owain, and wins back Lady Catherine, but in the process, the data detailing the route home are sacrificed and the English are committed to an interplanetary existence. The epilogue reveals that the captain who is reading the transcript is an Earth man of the future who is preparing to meet the descendants of these English crusaders in their first encounter with Earth since 1345.