All over the world, people are venting their fury at one another on social media. Dropping their friends, giving vent to their hatred, and everywhere behaving with incredible cruelty. Even Donna has found that her friend Hettie, with her seemingly perfect life and fancy house, has unfriended her. And now, all over the world, internet trolls are dying. As more and more people give in to this wave of bitterness and aggression, it's clear this is no simple case of modern living. This is unkindness as a plague. From the streets of London to the web cafes of South Korea and the deepest darkest forests of Rio, can the Doctor and Donna find the cause of this unhappiness before it's too late?