The Sisters of Plenitude are humanoid cats who run the New Earth Hospital in the year five billion and twenty-three on New Earth in the M87 galaxy, where humanity has settled after the destruction of the Earth.
In the episode ‘New Earth’ the Doctor and Rose visit New Earth, where the Doctor has been summoned to meet the Face of Boe – who may be captain Jack Harkness at the end of his very long life. In the hospital they discover that the Sisters have a dark secret; driven to desperation at their increasingly ineffective methods of disease control, they have been breeding living humans as test subjects as they seek cures for ever more deadly diseases. The infected humans escape and the surviving Sisters are arrested for human experimentation.
In the episode ‘Gridlock’, the Doctor revisits New Earth with Martha to find that the entire surface population has been poisoned by ‘Bliss’, and the survivors are gridlocked in an underground motorway. Novice Hame (Anna Hope), the last surviving Sister, who having received penance for her sins is now the Face of Boe’s nurse, reappears; she explains that she managed to seal off the motorway saving those on it and that the Face of Boe has kept it operational. But the survivors are trapped and unaware of the disaster that has affected their planet. The Doctor determines that the danger of ‘Bliss’ has passed and works with Hame to try to power up the systems to unseal the Motorway but there is not enough energy left. The Face of Boe sacrifices his life energy to the system to allow the survivors to escape. Before he dies, he has a final message for the Doctor: ‘You are not alone.’
Sheelagh Wells was the makeup designer for ‘New Earth’; Barbara Southcott was the makeup designer for ‘Gridlock’; Neill Gorton is the prosthetics and special makeup designer for Doctor Who; Rob Mayor is the prosthetics and special makeup supervisor for Doctor Who; Victoria Bancroft was the prosthetic makeup artist on ‘New Earth’.