“The Master”, also known as “Missy” in female form, is a renegade Time Lord from Gallifrey, regarded as the Doctor’s arch-enemy. Despite being childhood friends and attending the Time Lord Academy together, the Master’s lust for power eventually drives the Doctor away, instilling an enmity between the two. The Master is often hellbent on world domination and killing the Doctor, though the two have acted as allies a few times. Their relationship is a complex one, the Master being aptly described by the Tenth Doctor as his “arch-frenemy”. As Missy, she starts to mend her friendship with the Doctor during the time he is obliged to guard the vault in which she is imprisoned. The Doctor goes as far as setting her free, trusting that she is rehabilitated and posing a test for Missy’s goodness. During this test, Missy encounters her last male incarnation, who becomes furious at the idea of standing with the Doctor. Missy shoots her previous incarnation, triggering his regeneration into her; though, as a result, he also shoots Missy, leaving her unable to regenerate.