1999: Leaving her infant son behind, a young mother named Cassandra Schofield departs Bolton, seeking a better life amid the lights of London.
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.