"Florence" was an extraterrestrial criminal responsible for murdering royalty on a far-off planet. She came to Earth to hide from the Judoon, a group of intergalactic police-for-hire, and took refuge in Royal Hope Hospital, London in 2008. The Judoon followed her to Earth and used an H2O scoop to snatch the entire hospital from the planet and plant it on the Moon. They then systematically searched the building and catalogued everyone inside to narrow down their suspects.
"Florence" remained hidden from the Judoon's scans by consuming the blood of the Chief of Medicine, B. Stoker. This allowed her to pass off as human as her biology assimilated Stoker's blood. "Florence" then modified an MRI machine to unleash a powerful monomagnetic wave that would fry the brainstem of every living thing for hundreds of thousands of miles, enough to kill the Judoon, the entire hospital and half of Earth. However, the Doctor interrupted her scheme and tricked her into feeding on his blood.
When "Florence" assimilated the Doctor's blood, it unmasked the true nature of her genetic structure. Florence would no longer register as human to the Judoon scanners and Martha Jones convinced the Judoon to scan her again, and this time the old lady was revealed for what she truly was: a Plasmavore. The Judoon then swiftly executed her by incinerating her with their blaster pistols.