First in a two-part special leading up to David Tennant's departure from the role he has played for four years. At the end of November's adventure The Waters of Mars, the Doctor set off in the Tardis seemingly determined to meet his fate head on, carrying with him the Ood's warning that his song is coming to an end and the clue that 'he will knock four times.' Now, one of his greatest enemies has been reborn, on Christmas Day, and with both determined to cheat death, the battle rages from the wastelands of London to the mysterious Immortality Gate. As well as the return of John Simm as the Master, this episode sees the reappearance of Donna (Catherine Tate) and her grandfather Wilf (Bernard Cribbins), who takes on the role of the Time Lord's companion for this two-parter. As Who fans will know, this is not the first time Cribbins has accompanied the Doctor on his adventures - he also took his place alongside Peter Cushing's Doctor in the 1966 movie Daleks' Invasion Earth: 2150 AD. Other guest stars include Timothy Dalton, David Harewood and June Whitfield