It's 1736 in the Prussian city of Königsberg, now the Russian exclave of Kaliningrad, and Leonhard Euler, probably the greatest mathematician of the 2nd Millennium, is on a jog or something and decides to try to cross every one of the seven bridges over the river Pregel once and only once to prove his worth as a nerd. He failed. And subsequently he went home and drew up a snazzy mathematical proof as an excuse.