Victorian London is the setting of this "Doctor Who" Christmas special, which finds the time-traveling doc (David Tennant) arriving in the city on Christmas Eve, 1851, to investigate a string of murders -- only to encounter a familiar foe: the Cybermen. He also crosses paths with another Doctor (David Morrissey) -- whom he believes is a regeneration of himself -- and together they must battle their common enemy, the evil Mercy Hartigan (Dervla Kirwan).