10 Best Doctor Who Crossovers Ever
3. The Three Doctors
When the tenth season of Doctor Who kicked off in 1973, it did so with a special broadcast in four weekly parts. The special featured the Doctor's first three incarnations, all played by their respective actors, William Hartnell, Patrick Troughton, and Jon Pertwee.
In the special, the solar engineer known as Omega, who was the creator of the experiments that gave the Time Lords the ability to travel through time, seeks revenge! The Time Lords left Omega for dead in a universe composed entirely of antimatter, and it's up to the Doctors to stop him!
The Time Lords recruit the Doctor from three separate time streams to help. When Omega begins to drain the Time Lords of their power, the Doctors show up in the nick of time to help the rulers of Gallifrey.
The Three Doctors was the first Doctor Who crossover special to feature a Doctor's previous incarnation alongside the current actor. It's widely considered to be one of the greatest crossover episodes ever made. It was novelized a couple of years after the show premiered, which helped flesh out some of the episodes' details more clearly.