Well, that was quick. Shortly after the supercut of "Robot" that gave us Tom Baker's debut as the Fourth Doctor on Doctor Who, we now have a half-hour supercut of "Castrovalva", Peter Davison's debut as the Fifth Doctor that took place 6 years later. Sometimes the BBC is too good to us. "Castrovalva" was the first story of Davison's first season, and is a direct continuation from Tom Baker's final story the previous season, which saw the Fourth Doctor ignominiously fall from the top of a radio tower and shatter his body on the ground below. He generates into Davison right at the end in a cliffhanger.
"Castrovalva" spends a lot of time showing the new Doctor in a state of post-regeneration confusion and instability, the first time the show really dove deep into the topic. Russell T. Davies revisits this convention in his second season of the new show with David Tennant's new Tenth Doctor making his debut in an unstable state and spending the first half of "The Christmas Invasion" asleep.
"Castrovalva" was a major reset for Doctor Who. The show was no longer defined by the goofy humour of Baker, who often used to ad-lib lines when he got bored with the more banal dialogue he was given in the scripts. Baker was increasingly unhappy with producer John Nathan-Turner's approach to the show and felt he was getting sidelined, which drove him to finally leave the show. The show was getting unwieldy with a large number of companions in the TARDIS and the Fourth Doctor was starting to look like a tired babysitting having to hurd Adric (Matthew Waterhouse), Tegan (Janet Fielding), and Nyssa (Sarah Sutton) like a bunch of unruly kids, though the Doctor was always more unruly than the lot of them put together. Davison being younger changed the tone of the show and it now felt like the Fifth Doctor and his companions were a bunch of squabbling roommates living together in university housing. To watch the Fifth Doctor stories now is to get a window into some marvel surreal 80s nostalgia.
Script editor aka Head Writer Christopher Bidmead had ambitions to tell hardcore Science Fiction stories grounded in real scientific concepts and this was evident in his script for "Castrovalva" and the writers he oversaw for the rest of the season. The Fifth Doctor's debut season was one of the most ambitiously Science Fictional seasons that Doctor Who ever had. The drawback turned out to be that Bidmead's desire to have scientific concepts explained by the characters ended up sounding no different from the totally made-up gobbledegook that less science-literal writers from other seasons wrote for the show. Later, Bidmead would be replaced as Script Editor by Eric Saward, a more messy writer who was said to not particularly understand Science Fiction at all, and it showed in the seasons he oversaw, including the Sixth Doctor's (Colin Baker) entire run.
In an amusing bit of real-life meta "timey-wimey" surrealism, Davison was said to be Tennant's favourite Doctor when he was a child, and Tennent went on to marry Davison's daughter Georgia Moffett, creating a quasi-meta 2-Doctor household that gets a bit creepy if you think about it too much. At the rate the world goes, one of their kids could become an actor and end up cast as The Doctor in the future. Don't laugh. You know this could totally happen. Some of you strange people might even want it to.