The real story
The text on the stone is a priestly decree from 196 BC honouring the young king Ptolemy V, copied in three scripts so that priests, officials and Greek-speaking rulers could all read it. Centuries later it was broken and reused as building stone in a fort near Rashid. In July 1799, French soldiers rebuilding that fort pulled it out of a wall, and an officer realised what it might be.
The British took it under the 1801 surrender terms, and it has been in the British Museum since 1802. The English scholar Thomas Young cracked some of the royal names, but Jean-François Champollion worked out that hieroglyphs mix sound signs with picture signs. He announced it in September 1822, and the story goes that he ran to his brother, shouted 'I've got it!' and fainted.