Romana has to go home to Gallifrey. They have mixed emotions—on the one hand, Romana misses home, the Doctor wants to see if Leela's gotten Andred into bed yet, and K9 II will get to meet K9 I, but on the other hand, they won't be able to have sex anymore. So they decide to take the scenic route.
Unfortunately, they're too busy with the foreplay to watch where they're going, so they crash into a CVE (which stands for Technobabble) and fall out of the universe. Fortunately, they fall into a hot pocket universe, which is like the regular one but all the coordinates are negative, which makes no sense (0 is usually put at the center, so everything left/back/down from the center has negative coordinates), but whatever, you can tell it's definitely a hot pocket universe because space is green, because science.
Realizing they've survived the crash, and they have a good excuse to delay Romana going home, they're pretty relieved, until they realize they may never be able to get back out. So they land on the nearest planet, in the midst of a verdant forest. They quickly discover a crashed spaceship, where the human survivors (who also fell through the CVE) have set up a colony, and because no asshole Time Lord came by to reprogram their computer, they haven't regressed into Tesh and Sevateem tribes.
They meet the Deciders, three senior colonists who organize the settlers into trying to repair the spaceship while also growing food and stuff while also not being eaten by marshmen. Meanwhile, a group of youngsters called Outlers have rebelled against the system because fuck Thatcher, and they live as crusties scavenging food from the riverbanks where the man won't hassle them because of the spooky mist.
Since repairing the spaceship is their top priority, the Deciders value technical and mathematical ability above all else, and introduce the Doctor and Romana to the brightest of their younglings, who also happens to be the younger brother of the leader of the Outlers, and who wears a big gold star for being so smart, and who is Adric.
From here on, there's a bunch more good stuff, but there's also a whole lot more Adric.
Stuff, but also Adric.
Stuff, but also Adric.
Stuff, but also Adric.
But finally, near the end, Adric goes off somewhere to mourn his brother, we can get back to the plot.
The Alzarians who thought they were humans make their spaceship work and fly away to freedom, while the Doctor and Romana fuck off to never see Adric again. Little do they know… Sigh…
Matt Smith was only 3½ years old when he wrote this script, which makes it all the more impressive.
Years later, Big Finish would create the Divergent Universe arc, where a TARDIS team that everyone loves travel into a different universe, pick up an annoying new companion, and go through a series of stories that are all supposedly connected by some really badly misunderstood evolutionary biology but really aren't connected at all, and could easily have been set in the normal universe with no changes, and everyone breathes a sigh of relief when they return to the normal universe and run smack into the Doctor's archenemy, except that damnit they brought the alien companion back with them. But it's different because the Divergent Universe isn't green. Or maybe it is, since it's on CD so we can't see it. Bravo Russell.
Romana's obvious reason for her reluctance to return home, even if it's never actually stated, is where Paul Cornell got the idea that the Curse of Pythia prevents not just reproduction, but sex, but it also only applies on Gallifrey. Time Lords who want to be asexual like Marc Platt can stay home and loom up new cousins, but the ones who like to have fun like the Doctor's parents can just go to another planet and fuck like rabbits and have Time Tots.
If you ignore that it introduces Adric, Full Circle is a good example of how the Bidmead era tried to use SF ideas to drive interesting and different plots, almost like real science fiction. Some of the science is a bit silly, but it works perfectly story-wise, the whole bit about evolution and false memories and the secret of the marshmallowmen and the spiders and the human colonists and their spaceship. The plot is complex, but not hard to follow, and it manages to move along without needing a villain at all, much less a camp mustache-twirler. But you cannot ignore that it introduces Adric. Go on, try it. See?
Beyond the good ideas, the Deciders are good characters, and well acted, and Romana is in top form. But also, Adric.
Some great direction from Grimwade, helped out by solid sets and locations, and some of the best-realized monsters ever on Classic Who, and Kingsland is doing his synth thing much better than most of his 80s successors. But also, Adric.
In summary, but also Adric.
Then again, this was actually pre-season 19, so this was one of those stories in which Adric was not actually so bad all of the time...
...But also Adric.