Joanna Page (playing a guest character), David Tennant, Jenna-Louise Coleman and Matt Smith at the script read-through for the special.
The BBC has concluded complex negotiations which will allow the episode to be screened simultaneously alongside its UK broadcast (likely to be around 7pm). Aside from a few live sporting events, this is likely to be biggest global simulcast ever, certainly for a drama show. The episode will also be aired in 3D and will be shown in selected cinemas worldwide, although the details of that have not been revealed yet.
The episode will see the Eleventh Doctor (Matt Smith in his penultimate adventure) and Clara Osborne (Jenna Louise Coleman) joining forces with the Tenth Doctor (David Tennant) and Rose Tyler (Billie Piper) to confront the menace of the Zygons. John Hurt will also star as a hitherto-unknown 'version' of the Doctor, although what exactly that means remains to be seen. A trailer shown only at Comic-Con hints that other enemies and familiar faces may also put in cameo appearances.