Charlemagne was a French king. In 800, he was crowned Emperor of Rome. Charlemagne brought Europe out of the Dark Ages, restoring art and culture. According to the Tenth Doctor, his friends called him "Charlie."
In the year 800, Charlemagne held court in a castle in the Ardennes in Belgium. Attempts were made on his life. One food taster died of mandrake poisoning. A feast was being prepared when the Tenth Doctor arrived for a pivotal meeting that afternoon with Pope Leo III that would make Charlemagne the Roman Emperor.
History was nearly derailed when Momus abducted Charlemagne from his carriage en route to his castle. On Planet 12 of the Ridion Alliance, Donna recognised a lot of the abducted dinner guests but had never heard of Charlemagne. The Doctor warned Charlemagne about the soup when he got back and told him he should treat the strange events as if they were a dream caused by too much cheese the previous night. (PROSE: The Lonely Computer) The Doctor began to recount these events to Agatha Christie when she referenced Poirot, a Belgian detective of her own creation. (TV: The Unicorn and the Wasp)