Lady Peinforte was a 17th century English noblewoman from Windsor who was skilled in both archery and in the black arts. Her signature weapons were arrows with poison-tipped golden arrowheads. At some point, she fought against the Roundheads in the name of Charles I.
In 1638, she sculpted a statue of herself out of validium and named it "Nemesis," hoping it would grant her ultimate power. It consisted of a silver-colored validium statue of herself holding a bow and arrow. From the statue, which was capable of speech, Lady Peinforte learned of the Time Lords.
The Doctor removed the bow and arrow components from the statue and fired it into space aboard a rocket sled. Lady Peinforte kept the arrow, but the bow was lost. Seeking to reclaim the statue, she had her mathematician calculate when the Nemesis would return to Earth. When the mathematician told her its path would bring it back to the planet in the year 1988, she killed him and used his blood in a ritual to send herself and her manservant Richard Maynarde into the future. They took the arrow with them.
By 1988, a former Nazi named de Flores had obtained the bow. Lady Peinforte found that she weren't the only one seeking the statue, which had returned encased in rock in the form of a comet. In addition to the Doctor, Lady Peinforte had to contend with a group of Cybermen as well as with de Flores and his men. Although she initially used her gold arrows to kill several Cybermen, she and Richard eventually withdrew, planning to let the Doctor, the Cybermen and the Neo-Nazis kill each other over it.
During a lull in the battle which saw the Cybermen abscond with the statue, Lady Peinforte discovered that the Cybermen had moved it and placed it inside her own modern day tomb. As she and Richard entered the crypt, de Flores entered, having made an alliance with the Cybermen. Karl, the only one of his men to survive the encounter with the Cybermen, shot at them. To save his mistress' life, Richard gave de Flores the arrow, and he and Lady Peinforte escaped through a secret passageway.
Losing the arrow caused Lady Peinforte to slowly go insane and become increasingly reckless and desperate. When the Cybermen betrayed de Flores, gaining the statue and arrow (but not the bow, which the Doctor had stolen from the Nazis), they moved the artifacts to an abandoned warehouse. Observing a man hitchhiking, Lady Peinforte and Richard did the same, and ended up receiving a ride from an American woman named Mrs. Remington, who drove them to the warehouse, where Lady Peinforte had her final confrontation with her old enemy, the Doctor.
She demanded that the Doctor give her the bow so that she could make the Nemesis statue whole again, but he refused and (as part of a gamble to doublecross the Cybermen and destroy their space fleet), gave the completed statue to the Cyber Leader. Just as the statue became complete once more, an insane Lady Peinforte leaped into the rocket sled with it and was absorbed by its power.