The TARDIS lands in an alien tropical rainforest at night where the Doctor, Adric and Romana discover a set of rails stretching through the undergrowth. These tracks carry a long-crashed spaceship that’s been converted to run along them like a train. The ship has to keep moving because only the night-side of the world is habitable. The sun on the day-side burns so hot that everything on the surface is turned to ash. But the stress and strain of the constant movement is beginning to take its toll on the ship. Parts are starting to break down, and the relentless heat gets ever closer - but the greatest danger may be on the inside...