Doctor Who sees an exciting Magic Space Ghost, playing in a big old house. This is annoying for Doctor Who, for a number of reasons.
Partly it is annoying because it challenges his theory that space ghosts don't exist, and are just a thing stupid people believe in. He likes telling people that their beliefs are nonsense, and ghosts are excellent for doing just that.
Mostly, though, he is annoyed because it means he will now have to do an investigate, when he wanted to spend the weekend in a silk robe, drinking red wine and laughing at the Brigadier's moustache.
This story sees the introduction of best companion ever, Sarah Jane Smith. Here she is, looking both delightful and confused. She has accidentally gone back in time, and is clearly unsure why this has happened.
No mention is made of the fact that she must have been inside the TARDIS, and thus seen its massive, freakish 70s disco interior. She doesn't seem to have even registered it.
This is a welcome change from the last companion, Jo Grant, who was thrown into a panic every time she met someone in an unusual hat.
The Magic Space Ghost lives here, in the past, with these guys. He doesn't want to. He thinks they're morons, and he is right. They wear jumpers made of dead cats, and shout angrily all the time - even when they're really happy and having a lovely day.
Magic Space Ghost has landed here by accident and wants desperately to leave. I think he's persuaded these guys to let him do Science Experiments in their cellar. In return he lets them play with his Space Guns, and pretends to join in the awful workplace banter.
Sarah sneaks into the bad guys' castle with this guy - Hal the Archer. They're both doing excellent sneaking, I think, and deserve to get in.
Sarah is hoping that Hal will lead her to the people in charge, so she can ask them how to get back to normal times.
She's already sick of The Past - no-one bathes, everyone is a misogynist, and their selection of white wines is laughable.
Magic Space Ghost takes off his helmet and - surprise! - he's a horrible alien with terrible skin and a worse haircut. He's called a Sontaran, and his main hobbies are Science and Doing War.
He's reached a point in the adventure where he's thoroughly sick of everything that's going on. Other monsters get to do an invasion by this point, but there's only one of him, and so it would take ages.
Plus, he's in olden times, so what would he even invade? There's no shops to smash up, or landmarks to destroy.
It is stupid being him and he does not like it.
I can't remember what's going on here. I think the Main Bad Guy has tried to fight the Sontaran, and it has gone badly for him.
The Sontaran may look like he was drawn by a child who wasn't really paying attention, but he is excellent at hitting people in the face.
The Bad Guy is now trying to think of a way to backpedal. Maybe he could pretend that 'Toad Face' is Earth slang for "Really cool guy" and that hitting people with an axe is a mark of respect?
Doctor Who finally turns up and decides to get involved in the story. Sarah has changed into this rather fetching top and now looks like she's spending the weekend at a festival.
Doctor Who is anxious to take her away from all this, and get her up to speed on the basic rules of Being His Companion. These involve:
a) Telling him he is amazing.
b) Making sure he has first dibs on any snacks or alcohol they come across in their travels.
c) Ignoring the things other aliens say about him - they are all liars, and stupid faces, and jealous, and anyway, it was self defence.
d) Not minding if he suddenly tires of her and leaves her behind and gets someone new.
Doctor Who confronts the Sontaran, and says, "Stop invading the past. Also, your face is awful, and you have no friends."
The Sontaran finds this very hurtful, and expresses this by trying to shoot Doctor Who with his laser wand. Sadly for him, Doctor Who has got a special metal swirly thing, which makes the lasers bounce off.
Or something. I can't remember how it ends. I think the archer guy from earlier kills the Sontaran, and then Doctor Who runs off, shouting "I have won again!"