Henri Becquerel discovered radioactivity because the weather was bad. He had planned to test whether sunlight caused uranium salts to emit X-rays, but overcast skies forced him to leave his photographic plates in a drawer with the uranium sitting on top. When he developed the plates days later, they were fogged — proving that uranium emitted radiation on its own, without sunlight, without any external energy source at all.

This episode traces Becquerel from his physicist dynasty family through the accidental discovery, the shared Nobel Prize with the Curies, and the radiation burns on his chest that proved his discovery could be dangerous.

• The Becquerel physics dynasty — three generations of physicists leading to Henri's discovery
• The bad weather, the drawer, and the accidental discovery of spontaneous radioactivity
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