According to Digital Spy, the BBC have released the following info about The End of Time:
“It’s the Tenth Doctor’s final journey – but his psychotic nemesis the Master has been reborn, on Christmas Eve. With both determined to cheat death, the battle ranges from the wastelands of London to the mysterious Immortality Gate, while the alien Ood warn of an even greater danger approaching, as a terrible shadow falls across the entire Universe.”
… and ‘The End Of Time, Part Two’: “The Doctor faces the end of his life as the Master’s plans hurtle out of control. With the sound of drums growing louder, and an ancient trap closing around the Earth, the Doctor and Wilf must fight alone. But sacrifices must be made, and the deadly prophecy warns: ‘He will knock four times.’”
Sounding good, but what will knock four times?