I think it’s a display of the show’s increasing self-confidence that it’s able to start doing ongoing stories than run across multiple serials. We haven’t seen the Master for eight months and as many episodes. This picks up his storyline from the end of The Dæmons, with him in prison, but predictably plotting his escape. And while it might have been safe to do this as a Season Eight throwback, instead it’s a sequel to a Season Seven story minus UNIT, with the Royal Navy standing in (which means the Doctor is again in a slightly uncomfortable relationship with military authority, rather than the increasingly cosy partnership with the Brigadier).

In common with the last two serials, Pertwee’s performance feels a lot more relaxed than last year. His grouching at Trenchard’s officiousness:

TRENCHARD: You’ll enjoy talking to him.
DOCTOR: If ever I get the opportunity.

Would have been played with genuine annoyance last season, but this year he flashes a smile at Jo as he snarks, showing it’s all in good spirit. He seems cheeky and schoolboyish, just like he often was with Liz when faced with the Brigadier’s pomposity, rather than in a foul temper. Elsewhere, Pertwee seems to enjoy showing off his skills on the water, and smiles charmingly at the Navy ratings sent to detain him.

In fact, the episode is filled with great performances, both from the leads and even minor characters like Robbins, the sailor, who separately accepts bribes from the Doctor to borrow his boat and from Jo to borrow a motorbike. Elsewhere, Pertwee and Edwin Richfield (playing Captain Hart) have worked out a nice bit of business where the Doctor takes over Hart’s office and gives the captain permission to sit down. This all feels very well rehearsed, and you can see why Michael E. Briant kept being asked back to direct.

Delgado also seems to be delighted to be back, initially pitching his performance to suggest the Master is a diminished figure ‘trying to keep fit’ and ‘reconsidering his life’ as he’s no longer able to hypnotise his guards or plan to rule the universe. But the moment he’s not being observed by the Doctor, he’s back to his old self, enthusiastically planning his next scheme with the support of the stolid Trenchard (Delgado’s expression when Trenchard tries to explain children’s puppets is wonderful).

In among these, the plot – missing ships, a mysterious abandoned sea fort – bubbles along nicely in the background, and we get our first glimpse of a Sea Devil. It’s even more rubbery than a Silurian but, weirdly, is more effective and memorable, probably because it reminds me horribly of a dead fish head on a supermarket counter

Next episode: The Sea Devils – Episode Two