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.

And Dorothy is haunted by terrible nightmares, vivid dreams that begin just as scores of young runaways are vanishing from the dark alleyways of London. Could the disappearances be linked to sightings of sinister creatures lurking in the city shadows? Why has an alien satellite entered a secret orbit around the Moon?

Investigating the satellite with Ryan, Graham and Yaz, the Doctor is thrown together with Ace once more. Together they must unravel a malevolent plot that will cost thousands of lives. 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?