After three series and eight specials since taking over from Christopher Eccleston in 2005, David Tennant is finally handing over the key to the Tardis - but before he can regenerate into Matt Smith, the youngest actor to portray the 900-year-old Time Lord, there is still the little matter of doing battle with the Master once more. As their psychotic enemy's plans hurtle out of control, and an ancient trap closes around the Earth, the Doctor and Wilf fight alone. But sacrifices have to be made, and the deadly prophecy warns: 'He will knock four times.' David Tennant stars as the Tenth Doctor and guest stars include John Simm, Bernard Cribbins, Catherine Tate, Timothy Dalton, David Harewood and June Whitfield.