It's a long way back across the 'verse to Earth-central, especially with a target on your head. After the near-disaster on-board the Freya, and the hushed-up events of the Dellinger, the Doctor and Riley lie low on a private liner, planning to jump the next waygate back to the inner systems. They're used to hiding, and to running. But even they aren't prepared to be hijacked, and jumped across a hidden waygate to the edge of the 'verse. And what waits out there was never supposed to be found.