Which novel by C.S. Lewis is the final volume of the Space Trilogy?

That Hideous Strength is the final volume of C.S. Lewis’s Space Trilogy.

Published in 1945, That Hideous Strength follows Out of the Silent Planet and Perelandra. Unlike the first two novels, which focus more directly on journeys to other planets, the third book is set largely on Earth and presents a conflict involving a modern institution, scientific ambition, and supernatural forces.

The novel’s title comes from a line in David Lindsay’s 1920 science-fiction novel A Voyage to Arcturus. Lewis also drew on medieval and Arthurian ideas, including the figure of Merlin, while developing his criticism of attempts to use technology and organized power without moral limits.

The trilogy’s publication order is straightforward: Out of the Silent Planet appeared in 1938, Perelandra in 1943, and That Hideous Strength in 1945. The Dark Tower is an unfinished Lewis work, not the trilogy’s conclusion.