When the Master is in town, there really is only one thing to do.
You need to go to the circus.
Yes, after stopping Jo from blowing everybody up, the Doctor lets it be known that the Master has some hypnosis-type powers, powerful enough to make people do things that they normally wouldn’t. People who believe you can’t hypnotize people to do things they normally wouldn’t like Captain Yates are mostly just poorly informed. Now imagine saying that as rudely as possible, and you get the Doctor’s explanation.
While all this is going on, the Master is taking over a plastics factory. The son of the company’s founder is weak-willed enough to take over, and he does have a nifty self-inflating arm chair that has the added bonus of eating stubborn Irishmen who don’t know when to stop asking questions. Oh, and when the retired founder comes to investigate, he’s too mentally powerful, but the Master does toss a plastic monster doll into the back of the old man’s car, and that kills him later.
There’s something to be said about the series’ switch to color showing just how bad some of the show’s special effects are. There’s a lot of obvious blue screen effects here. And that’s not getting into killer chairs.
Good news, though, is that the Brigadier’s people found some dirt that actually leads the Doctor to the circus where he figures the Master must be hiding out. Sure, he could take some soldiers with him, but he’d prefer to go alone. That includes telling Jo to stay behind, but Jo hides in the back seat of Bessie, so she must be that sort of character.
Fortunately, this time she actually is helpful. See, the Master did take over the circus, and when the Doctor tries to get into the Master’s disguised TARDIS (looking as it does like a horse trailer), he’s caught by the ringmaster and the strongman. Jo manages to distract the guards, they find the missing scientist, and the Doctor even briefly gets inside the Master’s time machine to swipe…something.
And hey, the Brigadier noticed she was missing and sent people to back the Doctor and Jo up. That’s a good thing as the missing scientist kills himself to keep from blowing up the Doctor and Jo, and that leads to the two getting mobbed by circus folk.
Fortunately, the police show up and escort the Doctor and Jo away.
Unfortunately, they aren’t actual cops but Autons.