10 Best Doctor Who Crossovers Ever
5. The Two Doctors
While it's not atypical for the Doctor to cross paths with a former (or future) incarnation, it's usually done to save someone or something from total destruction. It's rarely to do with the Doctor himself, but that's exactly what "The Two Doctors" is all about, as the 6th Doctor has to save the 2nd to ensure his existence!
In the story, the 2nd Doctor and Jamie are on a mission given to them by the Time Lords to meet with Dastari, the Head of Projects, on a space station. The Dastari is on the verge of discovering the secrets of time travel, but the experiment could destroy the fabric of time.
The Sontarans attack, and the Doctor is killed, which brings the tale to the 6th Doctor who has a memory of his 2nd incarnation dying. To ensure that doesn't happen, he contacts the Dastari for help, but things don't go exactly as planned.
Ultimately, the 6th Doctor finds his 2nd incarnation, and the two work together to stop an alien species from stealing a secret of the Time Lords with the help of the Sontarans. If the 6th Doctor fails, he will cease to exist, so the two Doctors do everything they can to save time... and themselves!