When the Doctor and Rose stumble across thirteen-year-old Lily, they both agree she needs their help. Lily thinks there are monsters in the closet, hiding under the bed. And that they've taken her mother and brother - who went missing months ago. When asked about the monster, Lily can only say it's made of immense light and power. Rose and the Doctor must find out who and what the creature is, and where it's taken Lily's family. It is a search that will take them from the Cardiff Rift right back to the childhood of Lily's grandmother, and another disappearance all those years ago . . .