A drop of water falls on the head of Ashur-banal, the erudite but merciless king of Assyria, as he walks through his capital, Nineveh. Having dissolved into the atmosphere, it reappears in 1840 as a snowflake that falls into the mouth of King Arthur of the Sewers and Slums, the fancifully named son of a mudlark, born on the banks of the Thames.