The human immune system has no single organ; it runs as a distributed network across the whole body, and this DW documentary breaks down how it works, how it can fail, and how doctors are learning to turn it against disease. A pediatrician in Hamburg explains how immunity develops in infants and young children. Immunologists in Paris and New York discuss the links between gut microbiome and immune response, drawing on data gathered since COVID accelerated research into the field. In Heidelberg, an oncologist walks through immunotherapy treatments that train the immune system to target cancer cells directly, using patient cases and lab footage to show the mechanism at work. The film moves through four questions in sequence: what the immune system is, how it can be strengthened, when it breaks down, and how it can be harnessed medically. Interviews with international researchers are paired with clear visual explanations of cell-level processes, aiming to make current immunology accessible without oversimplifying the science behind cancer treatment.