The Sontaran is Field Major Styre, which I think is short for "Styrofoam" because Sontarans like things to be short, just as they are and as this story is. Roth tries to run away so Styre shoots him in the back because, as he tells Sarah, he is "a warrior."
Styre imprisons Sarah behind an invisible wall so when Harry tries to rescue her he bounces off it. Poor silly Harry. This kitten knows how he feels:
Styre makes his report to fellow Sontaran Marshal who appears on TV.
The Doctor comes to rescue Sarah, but after he sonics away the forcefield Styre comes and knocks him down. The Doctor tries to run away but he gets shot like Roth. Oh noes, maybe the Doctor is dead too?
The robot has captured the other spacemannys and it takes them to Styre. Vural gives away that he has been helping Styre, but Styre betrays the betrayer because he will not let Vural go even though he said he would.
The Doctor is not dead of course, and he teams up with Harry.
"Doctor, I thought you were both dead!"
"Not me. A piece of the sonastic locking mechanism from Nerva's rocket."
"Foresight. You never know when these bits and pieces will come in handy. Never throw anything away, Harry.
Now, where's my 500-year diary. I remember jotting down a few notes about the Sontarans... It's a mistake to clutter one's pockets, Harry."
Sending Harry to look after Sarah, the Doctor goes to find out what the Sontarans are up to. He meets the robot and disables it with his sonic screwdriver.
I don't understand exactly what Styre does with the spacemannys in the next scene. I think he is bribing them to squash Vural by offering them £30, then £200, then up to £500, and they try to resist being tempted. I'm a cat so I don't understand money. Styre is called away to report to Marshal so we don't find out how that turns out.
The Doctor has a plan to fight Styre while Harry
"Is that the Sontaran way? The mighty warrior sheltering behind his gun? I challenge you, Styre! Single combat! Or are you afraid?"
"Afraid..? A Sontaran, afraid? All right. Come to your death!"
Vural helps the Doctor and is killed, and Styre goes back to his ship for noms and a rest, while still trying to keep up his bravado and convince them (and himself) he is winning:
"I shall kill you all now... But first I have more important tasks to perform."
Field Major Styrofoam melts (a simple but effective - and quite horrible really - special effect) and his spaceship disappears in a puff of smoke. The Doctor speaks to Marshal on the TV.
"Styre! Your report, the intelligence... What is this?"
"Your Waterloo, Marshal."
Aha! Another excuse to post this video:
The Doctor, Harry and Sarah transmat away, leaving the surviving spacemannys behind on Earth. The transmat doesn't quite work first time though, providing the "lol" moment to end the episode on.
The Sontaran Experiment is a good story that uses the regular characters well and has a really horrible baddy in the shape of a Potato Head. Because it is only two episodes long it is fast-paced and doesn't feel at all padded or as if it outstays its welcome. It has some funny moments but some scary bits as well.
However, I feel as though there is something missing from it that stops it being a very classic story. Maybe it lacks the depth that longer stories have the time to develop, and certainly the spacemannys are underdeveloped as supporting characters.
Or perhaps I am just disappointed that, after the cliffhanger to part one relied on the reveal of the monster that was in the story's title, the rest of the story didn't feel nearly as Terry Nationish as that bit and suffered by comparison. It makes me want to watch a proper Terry Nation story soon. Anyway, I wonder what the next story after this one is...