On a misty mountain ridge at dawn, you raise your arm to shield your eyes and a hundred-foot faceless giant in the fog raises its arm too, its head crowned by perfect concentric rainbow rings. For centuries people read this as a god or a demon. It is the Brocken spectre, an atmospheric optical illusion that has manipulated human perception from the peaks of Germany to the Andes, and this episode pulls back the curtain on exactly how the trick works.

It breaks down the optics, your own magnified shadow projected on a cloud bank, the backscattering that produces the rainbow "glory," and the collapse of depth perception that makes the figure look colossal and pyramid-shaped. Then it follows the cultural afterlife: the witch-haunted Harz Mountains that gave the phenomenon its name, Carl Jung's fascination, and its appearances everywhere from a Japanese courtroom drama to anime and a metal album. The monster in the fog, it turns out, is always you.

• The geometry of the illusion: sun as projector, you as film strip, fog as a 3D screen
• Backscattering and the "glory": how water droplets bend a holy rainbow halo around your shadow
• Why it looks gigantic: spatial anchors, the featureless void, and a brain that loses its grip on scale
• The Brocken peak: 300 foggy days a year that turned an illusion into witchcraft and folklore
• From airplane windows to car headlights: how to see your own spectre, and why it demands introspection