Once, a girl called Ace travelled the universe with the Doctor – until, in the wake of a terrible tragedy they parted company. Decades later, she is known as Dorothy McShane, the reclusive millionaire philanthropist who heads global organisation A Charitable Earth.
Dorothy is haunted by terrible nightmares that begin just as scores of young runaways are vanishing from the dark alleyways of London – and just as an alien satellite enters a secret orbit around the Moon.
Investigating the satellite with Ryan, Graham and Yaz, the Doctor is thrown together with Ace once more. But can the Doctor atone for her past incarnation’s behaviour – and how much must Ace sacrifice to win victory not only for herself, but for the Earth?