Man, do not try to stick the Fourth Doctor in a trap. If he’s not unconscious, he will find a way out with or without the sonic screwdriver.
That happens here. Someone tries to gas the Doctor, and he gets out without too much trouble by using the sonic screwdriver.
But because bad guys never learn, that Cybermat managed to poison Sarah Jane.
However, the others find her in time, and the Doctor knows they can use the trans-mat to beam her down to Voga and clear the poison out of her system. He sends Harry along as well because someone has to go. The Doctor could, sure, but he has other issues to deal with.
For example, who sent that Cybermat around? Look, Doctor Who is not a subtle show. Of course it was Professor Dick. He got a lot of gold to do it, too.
Why is the gold supply on Voga such an issue? Simple really. Gold kills Cybermen. Is that new information? I don’t recall hearing that before. Point is, a planet full of gold would be a problem for the Cybermen.
It’s also a problem for Harry and Sarah Jane. It seems Professor Dick had a quota of humans he could keep alive on the Nerva, and the locals aren’t sure if Harry and Sarah Jane are throwing the numbers off. They capture the pair, though both escape and hide as the Vogans look for them.
As for the Doctor, he sees a Cyberman ship coming. There isn’t much the Nerva can do to stop it, so he goes down with the two other crewmen to try and repel the Cybermen boarders. The crewmen use some guns.
Wait, Cybermen are bulletproof. That doesn’t work. Instead, they zap the crewman and the Doctor. Since the Doctor was hit, it probably isn’t fatal. Sure, the Fourth Doctor is unconscious again, but stuff happens. And we still have two episodes of this serial left.