For nearly 200 years visitors to the British Museum saw a black basalt slab, but a 1999 cleaning revealed the Rosetta Stone is actually sparkly grey granodiorite with a pink vein, blackened by chalk, carnauba wax and London smog. This episode tells the biography of that object, starting in 196 BC Memphis, where a decree honoring the 12-year-old king Ptolemy V was carved in hieroglyphs, Demotic and Greek as a religiously sanctioned bribe to Egyptian priests during civil war and foreign invasion.

We follow the stone from an abandoned temple to a Mamluk fortress wall at Rashid, its 1799 discovery by French officer Pierre-Francois Bouchard, the 1801 British seizure after French scholars threatened to burn their notes, and its wartime hiding in a postal tube station. Then comes the decipherment race: Silvestre de Sacy and Akerblad on Demotic, Thomas Young on cartouches, and Jean-Francois Champollion cracking the phonetic system in 1822 using Cleopatra and Abu Simbel inscriptions. We close with plagiarism feuds, Zahi Hawass and the repatriation debate, and how Rosetta Stone became a universal idiom for unlocking mysteries.

• Why the decree granted tax immunities and gifts to priests to shore up a crumbling boy king
• How the stone was smashed, reused as a building block and rediscovered during Napoleon’s campaign
• The general who hid a 1,600-pound slab under carpets in his personal baggage
• Why three texts on one stone still took over 20 years to translate
• The modern tug of war between the British Museum and Egypt over ownership