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Sarah led Luke, Clyde, Maria and K-9 carefully through the halls of the Prime Minister's residence. Mr Smith had found evidence of Dalek interference in the British government, and she was here to see if this were true. Carefully she turned the corner, sonic lipstick at the ready, only to run into....
The Doctor (Tenth) and Donna, here investigating Harriet Jones' return to power, possibly bought and paid for by the Sontarans in their endless war against the Rutans. After many exclamations and confused introductions, they decided to join forces and continued to creep slowly down the hall towards the main conference room but before they arrived, they nearly collided with....
The Torchwood team. Jack, Martha and the rest had been called in by Harriet Jones to investigate temporal oddities and they had tracked it down to a rift caused by the Cybermen from the parallel Earth, attempting to reconquer this Earth. Of course, right behind them were....
Torchwood from the alternate Earth. Alt!Pete, Rose, Jackie, Mickey and Jake weren't about to let them succeed. And to this end, they followed the Cybermen through the rift. Many tears and complicated explanations later, they were about to continue, but not before they were interrupted by....
Brigadier Bambera, Brigadier (as he still preferred to be called) Lethbridge-Stewart (who had been pulled out of retirement to advise) and their crack UNIT troops (including Harry, Yates and Benton), who had discovered a possible attempt by the Master to reclaim the position of Prime Minister. Once the Doctor had finally convinced them that he was really the Doctor, and everyone had been introduced to each other, they were startled to hear the sound of a materialising TARDIS. Fearing it was the Master, the crack UNIT troops pulled out their weapons, only to have several women emerge from the TARDIS and glare at them....
Of course, it was Leela, Romana and Susan, who had narrowly escaped the destruction of Gallifrey with the help of Ace and her time bike and now were travelling the galaxy in search of adventure. They had picked up a signal from the Nestene consciousness, and traced it to Earth. Hugging, yelling and a few punches were thrown, and Sarah had to restrain Leela from attacking with her knife, while the Doctor explained to his current companions that yes, Susan was his granddaughter. Finally, they started arguing about how to split up and deal with the various threats when....
The Doctor's other incarnations started showing up with assorted companions. (Some were admittedly edited in with amazing CGI effects, allowing them to be played by the original actors.) The hall was getting incredibly crowded, and everyone was confused. When suddenly the Cybermen, Daleks, Yeti, Macra, Slitheen, the Master and the Rani all attacked at once, but they were astonished when the true mastermind was revealed to be....
The Crimson Chapter of the Orbus Postremo, following the manipulations of the Mandragora Helix. At which point Josh and Nat (played as in the audios by Lis Sladen's daughter) appeared and kicked arse. A long drawn out battle followed in which Ace's stash of Nitro-9 came in very handy. And finally the threat was ended with a special effects extravaganza that was the envy of many a sci-fi movie, involving explosions, an alien fleet, CGI aliens, transformations, and a spectacular flashback to the Time War.
A long and tearful reunion followed, before everyone went off to their proper times and places including Rose, who had grown up enough to admit that what she had felt for the Doctor was just a crush (and was now dating the Martha from her world and couldn't wait to get back to her).
Since RTD had slipped on a banana peel and was in a full body cast, the script was rewritten by Steven Moffatt (who unbeknownst to Rusty edited the original ending into something resembling coherency). The cameo appearance of Spike and Lynda from Press Gang was an unexpected bonus (especially when Spike was instrumental in stopping the Master, by tripping him as he attempted to flee).
The episode was adored by fans and critics alike...except the Rose/Doctor=twuwuv shippers, but everyone ignored them. It swept the awards for the next year and would be held up for years to come as a pinnacle of sci-fi programming.