Comets raining down on the Earth is the stuff of science-fiction movies. Yet recent mass extinctions have been caused by just such events, and, Daedalus has noticed, these seem to occur roughly every 26 million years. One theory to explain this is that the Sun has a companion star ('Nemesis') with a 26-million-year period — every time Nemesis swings past the Sun, it disturbs the Oort comet cloud, resulting in a storm of comets. To test this theory, Daedalus proposes investigating the age of craters on the moon, which should reveal the period of Nemesis and when she will strike next.