The Depository is a vast store of the literary remains of Earth's cultural greats: Charles Darwin, Martin Luther, Wilkie Collins, Barbara Cartland.
Benny's heard that there's a job going, and thinks that it might offer just the kind of stable environment that her son requires. It has friendly bars, a reliable atmosphere shield, a fantastic patisserie run by a robot called Mrs Tishpishti — and no history of alien invasion.
But you know where you are with an alien invasion. Or you do, at least, if you're Bernice Summerfield. And even she has never encountered a monster whose main objective is to tell her that she doesn't look very good in trousers.