The Zygons are a race of shapeshifting humanoids from Zygor, a planet destroyed in the Last Great Time War. The Zygons have a biotechnology called body-prints that boosted their natural shapeshifting abilities, functioning as a bridge between the Zygon and the being they wish to emulate. It works by keeping the “original” individual a Zygon wanted to emulate in a stasis in machine, which the Zygon could later access to emulate the shape of any being hooked into it. In 2013, when a group of Zygons infiltrates UNIT, Kate Stewart threatens to detonate a nuclear device to prevent them to access the alien technology stored in the Black Archive. The War, Tenth and Eleventh Doctors intervene, erasing the memories of the humans and their Zygon doppelgangers, so that nobody can know who is human and who is Zygon, making sure that both parts could be treated fairly and equally; and so, they finally reaching a truce between the species. After the agreement, it is decided that the Zygons are allowed to live on Earth on the condition that they disguise themselves as people and live incognito.