‘It’s called compassion Doctor, it’s the difference that remains between us.’ Steven Moffat did the whole ‘She cares so I don’t have to’ storyline with Peter Capaldi and I like this about the same. In principle, the idea of the Doctor going bad is workable – The Invasion of Time showed that. But the regeneration has already disrupted normality enough, and, thanks to the decision to end the season with this, there are only four episodes to establish the new reality. So, keeping the audience so uncertain about the new Doctor for so long is a risky gamble.

As a vehicle to show off Colin Baker – it works. He gets to play bombastic, scared, angry, ruminative and whimsical. I dislike some of the stuff he’s given to play (calling Peri a ‘stupid girl’ is unnecessary), but it’s hard not to enjoy quite how much he throws himself into it. There’s a moment where he leaps across the room to strange Azmael which reminds me of the climactic confrontation between Peter Cushing and Christopher Lee in Dracula, and it’s rendered less horrid than the equivalent scene in Part One by virtue both of Azmael treating it like a mild inconvenience, and the Doctor immediately being thoroughly repentant. Essentially, I don’t like the premise of the story, but I don’t hate the way Baker plays it.

It’s still in no way any good, though. The plot itself is ok, but as thin as they get. The whole first half of the serial has been a wild goose chase that’s just delayed everyone arriving at Jaconda, and this episode pushes its luck further by parking the Doctor, Peri and Hugo in the TARDIS for a decent chunk. The revelation of Mestor’s plan here comes not because the Doctor has followed a trail of clues and revealed the truth, but because Mestor has finally got round to inspecting the kidnapped twins and instructing them on their roles. There’s a lovely moment when the Doctor explains the history of the giant gastropods, but it doesn’t help fill in the gaps or point the way to a solution, it’s just a bit of colourful backstory. It’s a shame that, given he’s a giant slug, Mestor didn’t remain in stately obesity in his throne room like Jabba the Hutt, but instead has to be carefully guided about by two Jacondans which tends to remove any marginal menace he ever had. The Doctor says to Hugo, ‘You even have a gun to enforce your will upon others’ as if he wishes he did too. Oh dear.

Next episode: The Twin Dilemma – Part Four