It's a good story, really.
That is to say, the first two episodes work very well, and then it all gets dodgy.
It's well designed; there are good sets and uniforms (interesting that those Terran Federation helmets remained in use for so long...), and it's well acted. Peter Davison is at his best; 'Professorial' is a word much used in 'The Making Of', and Mark Strickson gets to do some proper acting - first time since Enlightenment he gets to do the real stuff, rather than just buttoning his blazer in an interesting way.
Peter Gilmore is a bit one note (and the note seems to be James Onedin in a bad mood), but that's OK because it's easy to concentrate on William Lucas, and Lesley Dunlop (I find it so easy to concentrate on Lesley Dunlop) and Jeff Rawle - it's nice that Plantagenet doesn't go bananas and the whole story just become about him, because it looks like it ought to, then (as CH Bidmead explains) 'it all turns upside down'.
We meet Tractators. How to ruin a Dr Who story with one ill-considered production decision. It really is one of those, 'It's going so well, but it's a JNT story so they've got to louse it up somewhere...' moments: This time with woodlice.
They employed dancers to play Tractators, because dancers can move so beautifully, and then they stuck them in rigid costumes that didn't move at all. I can only imagine what colour the dancers turned the air, because it's an absolutely moronic error - like it was taken by the Gumbies on Monty Python - it is in the same ballpark as chaining Anna Pavlova into a sack and then chucking her onto the stage at Covent Garden and demanding she perform the Dance of the Sugar Plum Fairy. You can almost hear the 'Duh-uh-huh' of the dunce making the decision, and it's of such a fatal stupidity because if the Tractators had worked, the intrinsic cack-handedness of the concept might - just - have gone unnoticed.
Because that's the real problem; it's not just that Tractators *look* silly, they don't make sense. Man-sized woodlice that can affect gravity so that they can pull meteorites out of the sky? You what, Mr Bidmead? I don't believe that a creature of that size could exert that kind of pull - I mean, *physics*! Dragging people underground, that I can buy, maybe, but meteorites? No, and I don't believe in them pulling the TARDIS apart either.
And then there's the question of Tractator intelligence, the Doctor says 'These creatures must be really intelligent', blowed if I can see what evidence he's basing that on - are they curing the Common Cold? Writing 'Remembrance of Time Past'? Designing helicopters? Enjoying erudite conversation? The Gravis certainly isn't the brightest beetle in the box; 'Oh no, please don't pull my TARDIS back together' is neither a subtle nor an original gambit - Brere Rabbit uses it to very good effect in The Story of the Tar Baby but for 'Please don't throw me in the briars' to seem a Machiavellian stratagem, you really do need to be no older than about five.
So, good design, good acting, good direction, but silly monsters, and it's the monsters the whole story hinges on. Poor Mark Strickson; all that lovely 'Tractators!!!' acting, and then they turn out to be, well, Tractators.
I can only wonder that they didn't learn from this, so that when someone said 'Giant slugs' they'd have replied 'Oh no, not again', but as we all know, that's not what they said at all.