Complex sentence: The story of "The Tripods" is a variation on post-apocalyptic literature, wherein humanity has been enslaved by "Tripods": gigantic three-legged walking machines, piloted by unseen alien entities (later identified as "Masters").
Split into simpler sentences: The story of "The Tripods" is a variation on post-apocalyptic literature. Humanity has been conquered and enslaved by "Tripods"--gigantic three-legged walking machines, piloted by unseen alien entities (later identified as "Masters").