The Thousand Year Thaw

Once every thousand years, the frozen planet of Zycros thaws, revealing the most fertile soil in the universe. For a brief window, Zycros becomes the farming capital of the cosmos, and that brings with it many visitors - including a small blue box and its two occupants, keen to stretch their legs. But as the ice melts and the temperature rises, the Doctor and Peri realise that this pleasant stroll may be the last one they ever take together...