Episodes that we see from Donna's perspective in the alternative world include: The Runaway Bride, Smith and Jones, Voyage of the Damned, Partners in Crime and The Sontaran Stratagem/The Poison Sky.
So that's why Lucius thought Donna had "something on her back" in The Fires of Pompeii!
Travelling Light Years
As the constellation of Orion is 93.6 light years away from Earth, either it vanished 93.6 years ago (as it would have taken that long for light from its stars to stop reaching us) or mysterious forces are at work...
Trick Or Treat
The Doctor refers to "the Trickster", a powerful being who appeared in the Sarah Jane Adventures story Whatever Happened to Sarah Jane?
The Doctor once disguised himself as a gypsy fortune teller in 1967's The Underwater Menace and former companion Tegan encountered one in a market on Mannussa in 1983's Snakedance.
The Doctor 'died' once before - in the 1996 TV Movie. Normally, regeneration is instantaneous, but there was a gap of several hours between the Seventh Doctor's demise on the operating table and his rebirth in the morgue.
Sarah Jane Smith once had a massive, invisible creepy crawly on her back - in her case a giant spider from Metebelis III from 1974's Planet Of The Spiders.
A circle of mirrors was used to expose another parasitic menace in 1982's Kinda - in that story, the manifestation of the Mara as a giant snake.
Chipo Chung, who plays the mysterious fortune teller also appeared in episode 11 of Series Three, as Professor Yana's faithful assistant, Chantho. Joseph Long, who plays Mr Colasanto, also appears as Luigi in BBC drama series Ashes to Ashes.
Compiled with the help of Peter Ware.