2004: Despite the best efforts of the time-travelling Doctor, 'Cassie' Schofield dies on Dartmoor, a vampirised victim of the sinister organisation called The Forge.
2021: All grown up, and a nurse at St Gart's Hospital, Thomas Hector Schofield – known as 'Hex' – meets, and becomes a companion to, that time-travelling Doctor… but remains unaware that his alien friend knew his mother, and watched her die.
1854: In the Crimean War, Hex takes a bullet, and is seriously injured. The Doctor promises to return him to St Gart's.
2025: Now. In a London ravaged by a deadly contagion… destiny awaits.