Horizon, the BBC's long-running science strand, turns its attention to why the brain can be tricked so easily by what the eyes and ears report. The film moves through optical illusions that bend straight lines and distort proportion, and auditory illusions that make a sound seem to rise in pitch forever or come from somewhere it isn't, using visual demonstrations to let the tricks work on the viewer directly before anyone explains them. Neuroscientists and psychologists then walk through the mechanics: the shortcuts the brain takes to build a fast picture of the world, and the cognitive biases that make those shortcuts fail in predictable ways. Rather than treating illusions as curiosities, the film uses them as evidence for how perception actually works, arguing that seeing is less a direct recording of reality than a construction the brain assembles and sometimes gets wrong.