Our immune system protects us against pathogens, harmful substances, and malignant cell changes, making it as indispensable to us as our heart or lungs. But it can also turn against us and be potentially harmful. This scientific documentary offers an insightful and entertaining view of the human immune system, focusing on four key questions: What is the immune system? How can we boost it? Under what circumstances does it fail? And how can it be used to cure diseases like cancer? It presents intriguing examples, graphic explanations, and interviews with international experts, such as a pediatrician who explains how the immune system develops in infants and young children, an immunologist who sheds light on the relationship between the microbiome and the immune system, a cardiologist who explores the connection between stress and the immune system, and an oncologist who uses immunotherapy to systematically combat cancer.