Estella was the wife of Orsino and, unbeknownst to him, an alien from an unknown planet. She had disappeared in 2194, apparently drowning herself in the main canal in Venice, and as she did this she cursed the town and Orsino. In fact she didn't disappear but stayed in Venice but went by the name of Ms Lavish. One hundred years later, she met the Eighth Doctor and Charlotte Pollard when they arrived in Venice.
At Orsino's end of the world party, she sat at the throne and it was revealed that she was Estella. The curse was a side effect of the technology of her people. On her and Orsino's self-sacrifices, the curse was reversed, and Venice was saved. (AUDIO: The Stones of Venice)