Castrovalva

If you're a regular viewer of the new Who, but haven't seen much of the old stuff, you might have noticed that the Doctor tends to be a bit... off his game after a regeneration.  He can be somewhat incapacitated or otherwise trying to find his footings.  Things are no different for the first episode of the Fifth Doctor.  The Doctor spends most of the first half of the story looking for his 'quiet room' on the TARDIS.  Okay, they call it a 'Zero Room' but it's basically a place where the Doctor can rest, recover and see how he's turned out this time.  Only the Master has set a trap and, to escape, the Doctor must jettison 25% of the mass of the TARDIS.  Since he can't pick and choose, the Zero Room is part of the TARDIS that drew the short straw and is gone.  Somewhere, somehow in here, we've lost Adric, who is a prisoner of the Master now, so Teagan and Nyssa manage to find a place in the TARDIS' databanks called "Castrovalva" that is not, contrary to popular belief, a vampire's reproductive organ.  It's a place where the Doctor can get the R&R he needs to be his old, er, new... his self again.

The girls build what looks like a coffin out of the door to the Zero Room and transport the Doctor to Castrovalva where he finally starts to act more like the Doctor again after an inexplicable brief stint of amnesia.  Once pieces start falling into place, the Doctor realizes Adric is missing and tries to leave, which ends up being harder than you'd think...

So we had a Doctor who was mostly trying to find his footing while the regeneration finished this story.  By the end, the iconic celery is attached to his lapel, he's got his 3 companions and we're off to the PROPER first adventure with our new Doctor!