‘This is Terminus: no-one’s happy here.’ I think this is easily the best episode of the story because the themes of freedom and self determination suddenly come into focus as the Garm, the Vanir and finally Nyssa assert their independence, and hope conquers despair. Whether that’s enough pay off to probably the most joyless Doctor Who so far is debateable, but it at least feels like the slog had some purpose.

Again, it’s a long way from perfect: too much of the episode features various characters struggling with handles (variously the Doctor and Kari, Olvir, the Garm, Tegan and Turlough); Tegan and Turlough might as well have stayed in the TARDIS, and – in a foreboding sign – Mark Strickson is given pretty much nothing to work with. However, the second half of the episode is strong. The Garm turns out to be far from a chained beast: he saves the universe, and in return only asks for his freedom. In a lovely moment, the Doctor smashes his metaphorical chains, then does the same for the Vanir when he breaks their dependence on the Company.

Meanwhile, Nyssa is cured, but that’s not enough. Her decision to stay and put her knowledge to good use is somewhat similar to Romana’s departure, but it’s also more emotional because of the risks involved. By all accounts the regulars were dismayed by the way the farewell scene was recorded, but it’s still the most touching since Sarah Jane’s, with a weight to it quite lacking in any of the hasty exits Graham Williams oversaw, and with some relatable human emotion that was, by their nature, not evident in the Doctor and Romana’s goodbye.

This is obviously inferior to Gallagher’s previous script, Warriors’ Gate, while sharing many of its themes of freedom from slavery, and people trapped in mundane work. Nothing here can match the poetry of the ruined castle and time-travelling mirrors; the imagery of the void, and the monochrome world (although most of this looks almost equally greyscale). But many of its ideas are as pungent, and it adds some heft to a season that’s too often been floating along on vague conceptual threats and fan service.

Next episode: Enlightenment