 In the episode , the Doctor — a time travelling alien played by David Tennant — and his companion Martha Jones ( Freema Agyeman ) are trapped in the past and try to communicate with a young woman , Sally Sparrow ( Carey Mulligan ) , to prevent the Weeping Angels from taking control of the TARDIS . Sparrow and her best friend 's brother , Larry Nightingale ( Finlay Robertson ) , must unravel a set of cryptic clues sent through time by the marooned Doctor . 
