As the Doctor, blind, stands at the entrance to a large vault asking for Missy's help, we see in flashback how she came to be there. Missy has been sentenced to execution and the Doctor, as a fellow Time Lord, has been selected as her executioner. Kneeling before her fate, Missy appeals to the Doctor's friendship and promises that she can learn to be good, "without hope, without witness, without reward". The Doctor, in his role as executioner, vows to guard her body for a thousand years in the vault, but the vow doesn't specify that her body will be dead.
