They do sound very similar though, which is why I got confused last time.
Barbara, Altos and Sabetha visit Kala to look for clues. Kala is really a baddy in league with Eyesen (who you may remember was the really obvious baddy in Sentence of Death) but she fools them by crying.
As soon as they go away Kala makes an evil face. She is the one who has kidnapped Susan. Unfortunately for her, Kala gave herself away and Barbara realises she is the kidnapper and they catch her before she can kill Susan.
Barbara 'phones Tarron and he arrests Kala, but Kala pretends that Ian is her friend instead of Eyesen so Ian is not saved yet.
The Doctor and Tarron catch Eyesen trying to get the key, which Eyesen hid inside the murder weapon and then Tarron put inside a cupboard, so he is arrested and Ian is freed.
Ian, Barbara and Susan teleport away.
Then the Doctor teleports away too. They leave Tarron behind because he doesn't have a teleport bracelet. Otherwise he could have gone with them and had adventures.
Back at Arbitan's place, Arbitan is dead and Yartek (the Voord leader) has stolen his clothes and captured Altos and Sabetha off-screen. Yartek has all of Sabetha's keys but not the last one because the Doctor still has it. He threatens Sabetha and Altos tells him about the Doctor.
The Doctor and Ian meet a Voord and the Doctor knocks it out with his stick.
Ian and Susan see Yartek, but he is cunningly disguised as Arbitan with his hood up. He impersonates Arbitan so perfectly that he gets the last key from Ian.
Actually his impersonation was rubbish! Ian saw through it straight away and only gave him the fake key. Yartek puts what he thinks is the last key in the machine and it blows up. The Doctor rescues Altos and Sabetha and they all run away.
The Doctor, Ian, Barbara and Susan all go off in the TARDIS after saying goodbye to Altos and Sabetha, who are staying on Marinus to have more adventures with Tarron!
That is how this fabulous and magnificent story ends, with a happy ending. I know the Doctor has more adventures with Barbara, Ian and Susan, but I don't know of any more stories that have Tarron, Altos and Sabetha in them. Maybe I will have to make them up myself?