“We don't get aliens in Sheffield.” In a South Yorkshire city, Ryan Sinclair, Yasmin Khan and Graham O'Brien are about to have their lives changed forever, as a mysterious woman, unable to remember her own name, falls from the night sky. Can they believe a word she says? And can she help solve the strange events taking place across the city?

On the edge of a Norwegian fjord in the present day, the Doctor, Ryan, Graham and Yaz discover a boarded-up cottage and a girl named Hanne in need of their help. What has happened here? What monster lurks in the woods around the cottage - and beyond?

As Graham, Yaz and Ryan return home to see friends and family, they find themselves haunted by very different experiences. Who is the figure calling from beyond the stars for help, and why? And what are the fearsome Chagaskas terrorising Aleppo in 1380? To find the answers, Team TARDIS must embark on a mission which forces them to face their darkest fears.