"The High Crusade" is a science fiction novel by Poul Anderson, published in 1960. The story is set in medieval England and follows a group of knights and peasants who accidentally encounter an alien spacecraft. When the aliens, known as the Hrossaks, attempt to invade Earth, the humans fight back, leading to an unexpected alliance and a series of adventures across the galaxy. As the knights and their followers travel to the alien homeworld, they face various challenges and cultural differences. The novel explores themes of heroism, faith, and the clash of civilizations, blending elements of both medieval and science fiction genres.