The Doctor has a companion or two (sometimes three) for a wide variety of in-story reasons. “The Deadly Assassins,” set on the Doctor’s home planet of Gallifrey, has none for in-story reasons. I don’t know why but I think I heard it was because Tom Baker might have thought it was a good idea.
It turns out it isn’t, and for a very good, out-of-story reason.
See, there’s a simple reason why the Doctor generally has a companion: he needs someone to talk to. Yes, the companion probably isn’t as smart as the Doctor, but that just means he has someone to explain things to. And by explaining things to a companion, the Doctor explains things to the audience. And the audience should, you know, generally know what’s going on most of the time. Sometimes, yeah, keep them in suspense, but if they never know…
Now, there does seem to be a solution to this problem. Basically, the Doctor spends a decent chunk of this episode talking to himself.
He’ll have a regular companion again next serial.
As for this serial, the Doctor returns to Gallifrey. Once there, he gets a vision of the President of the Time Lords Council dying at an assassin’s gun. Of course, the TARDIS just materializing inside the building alerts security. They are not happy to see an older model TARDIS just appear.
Interestingly, they do have notes on the Doctor’s last trip to Gallifrey, the one that ended with the Doctor’s exile to Earth. Interestingly, the Doctor was a member of the Council himself.
However, there is an assassin, a skeletal-looking guy with a sidekick. The whole thing seems to be set up to frame the Doctor from the looks of things. The Doctor does indeed get to the assassin in time, sort of. But not before a gunman kills the one guard who found the Doctor.
Oh, and when the Doctor finds an empty perch for an assassin, complete with rifle, he doesn’t seem the real killer is in the crowd. And you know, getting caught with a gun from a hidden perch is a good way for people to think you might be a killer…