Dr Michael Mosley continues his exploration of the brutal history of experimental psychology. Experiments on the human mind have led to profound insights into how our brain works - but have also involved great cruelty and posed some terrible ethical dilemmas. In this film, Michael investigates how scientists have struggled to understand that most irrational and deeply complex part of our minds - our emotions. Michael meets survivors - both participants and scientists - of some of the key historical experiments. Many of these extraordinary research projects were captured on film - an eight-month-old boy is taught to fear random objects, baby monkeys are given mothers made from wire and cloth, and an adult is deliberately violent before a group of toddlers. Michael takes part in modern-day experiments to play his own small part in the quest to understand emotions.