Our nearest large neighbor looks like a serene spiral in the night sky. But this calm is deceptive: Andromeda harbors violence within, and the consequences of its presence may touch us too. Today we'll examine the most stunning image of another galaxy in all of history — ten years of observations, hundreds of frames, two hundred million individual stars. From this picture we'll try to reconstruct Andromeda's turbulent past: what it did to its neighbors can still be read in its outlines. We'll unravel a riddle that gives scientists no peace and hints that we're missing something about the very nature of gravity. And we'll peer into the future — the very one where two galaxies come face to face. Except that fresh calculations have unexpectedly cast the familiar scenario into doubt. Watch until the end: what awaits us turned out to be far less predetermined than is commonly believed.