The Doctor, Liz and the Brigadier investigate the alien ‘spores’ – blobs of deadly mirror matter from the dawn of time. Liz summons fellow scientists to help the investigation. But one is secretly working for the BNKR, an organization preparing to take over the world. The BNKR unleash a killer spore. Meanwhile, the Doctor and Liz race to stop another spore, locked in a countdown to destruction. Can the Doctor and his friends prevent the entire Earth being converted into mirror matter?