It's Oliver's first trip in the TARDIS with the Doctor and Steven, and the three of them end up on a spherical space station which appears at first glance to be a giant greenhouse. Upon further inspection, Steven finds that they're orbiting Earth--the Earth of his past and Oliver's future, with its sky clogged full of detritus and debris. Steven knows how dangerous this can be for space traffic, but the aliens who run the station don't seem to ... Or do they? That, at least, works.