Alexander Fleming did not discover penicillin through careful, methodical research. He discovered it because he left petri dishes unwashed before going on vacation, and a stray mold spore drifted in through a window and killed the bacteria growing on one of them. The discovery that would save more lives than any other in medical history was, by Fleming's own admission, a happy accident.

This episode traces the story of penicillin from Fleming's famously messy lab bench through the decade of neglect that followed, and the desperate wartime effort by Florey and Chain that finally turned a curious observation into the miracle drug that changed medicine forever.

• Fleming's notoriously messy laboratory habits and the lucky accident of the contaminated plate
• Why Fleming himself failed to develop penicillin into a usable drug for ...