22 Edward Grove was a sentient Edwardian house, alive only during the moments of the chimes of midnight. When the Eighth Doctor and Charley Pollard went to the house on 25 December 1906, they created a paradox which made the house stronger. Typically such moments of sentience were temporary events caused by traumatic events, such as a murder, but the presence of the Doctor and Charley unintentionally created a temporal paradox. Scullery maid Edith Thompson would go on to work for the Pollard family in the 1930s, but she would commit suicide after Charley's apparent death in the R101 crash, only for Charley's appearance in 1906 to essentially remove Edith's reason for killing herself.
As a result, Edith Thompson's death became a focus of a two hour time loop 'inhabited' by the residents of the house, each of them trapped in a loop where some of them would die at the moment that the clock struck. At midnight, the people came to life again, and the loop started again.
When the Doctor and Charley convinced Edith that her life had meaning, the paradox and the time loop were broken and Edward Grove was defeated. (AUDIO: The Chimes of Midnight)