10 Best Doctor Who Crossovers Ever
9. "The Stolen Earth" & "Journey's End"
Doctor Who's 750th episode kicked off a two-part crossover featuring the characters of Torchwood and The Sarah Jane Adventures. "The Stolen Earth" and "Journey's End," closed out the fourth series as well as several dangling plot threads from earlier episodes.
The crossover begins when Earth and 26 other planets are stolen by the Daleks with Davros's help. The Doctor and Donna try to locate the missing Earth as are then contacted by Jack Harkness, Martha Jones, Sarah Jane Smith, and Rose Tyler, all with the hope of mounting a defense against the Daleks.
"The Stolen Earth" closes out with the Doctor taking a blast from a Dalek, which begins his regeneration cycle. Fortunately, he was able to divert his regeneration energy into his severed hand, halting the process. Doing this helped heal him completely without requiring any change into a new Doctor.
The episode features Donna's final regular appearance, but that only comes after she absorbs some of the regeneration energy, which gives her many of the Doctor's mental abilities, making her into "Doctor-Donna." The strain ultimately required all the Doctor's memories to be stripped from her mind, or she would have died.
The event also created an entirely human clone of the Doctor from his hand, which occurred when Donna touched the hand. At the end of the episode, that Doctor went off to enjoy a 'happily ever after' with Rose in an alternate dimension.