The ramifications of this treaty are explored in the ninth series two-parter "The Zygon Invasion" / "The Zygon Inversion" (2015). Zygons were allowed to re-home on Earth on the condition that they disguised themselves as people and lived incognito. In the intervening time, they developed the ability to retain a person's likeness after the death of the original and shapeshift into someone based on a telepathic scan of a nearby being. Although older generations of Zygons were committed to integration with human communities, the younger generations resented being forced to live as humans and quickly radicalised. The radicals and UNIT once again enter stalemate at the Tower of London, both poised to destroy one another, but the Twelfth Doctor makes an impassioned plea and convinces the radical Zygon leader to understand the lasting peace which the treaty was written to preserve.