[1] The story begins when a 14th-century English army is abducted by aliens of the Galactic Federation to serve as mercenaries on planets where only low-tech weaponry is legal.

The Galactic Federation was originally started over one hundred fifty thousand years ago by 3 races, with membership of the council requiring development of a form of faster-than-light (FTL) drive.

The story then moves forward to the 22nd century, on Earth, now under the single Solar government, where humanity has become capable of building vast ships in space and has developed an FTL drive.

For centuries, they have prepared and lay in hiding so could return at the moment of Earth's greatest need (just like the sword Excalibur in the legend of King Arthur, which gives rise to the title of the book).

But, with the technical superiority of the Avalon empire and a carefully planned series of rebellions on "protected" worlds in the Federation, humanity at least stands a chance.

Publishers Weekly praised Weber's "apt characterization" and knowledge of medieval weaponry and tactics, but found the emphasis on action meant that subplots were neglected.