It certainly looks like this is supposed to be a fairly serious SF retelling of the tale of Theseus and the Minotaur, but somewhere along the line they've thrown in a bunch of jokes; someone's decided it would be funny for Malcolm Terris's character to bellow "Weakling scum!" at the Anethans, someone else has decided it would be funny to see K-9 literally buried in tape (all of which is apparently the full damage report for the TARDIS), a third someone has decided it would be funny to hear comedy sound effects when the TARDIS fails to work properly near the start of part two... It's almost as if, collectively, the cast and crew decided, "Eh, this is the story going out over the Christmas holidays, let's just screw around and have some fun with it." And so Tom Baker appears to be delighting in every silly moment he can interject (blowing down K-9's "snout" to bring him back to life is just one example), while Graham Crowden, playing the Skonnon scientist Soldeed, appears to be seeing how long he can go without ever blinking (the answer is a long time, although he does blink occasionally). Yet seeing the two of them together is weirdly an exercise in underplaying, as they both try to deliver their lines in the most matter-of-fact manner possible.
The one exception to this pantomimic approach is Lalla Ward; she seems to have realized that no one is taking things seriously and stepped up to address the issue. Not that Ward typically plays Romana for laughs, but here she easily steps into the Doctor role, treating everything seriously as she confronts Soldeed and then leads the Anethans through the Nimon's Power Complex, and it looks like she's relishing the opportunity to play the straight leading role.
And that's the thing; based on these first two episodes, one gets the impression that former script editor Anthony Read's script was meant to be taken seriously (even if, so far at least, it suffers the same problem as Underworld, in that we're not given an independent reason to care that Doctor Who is retelling Greek myths), but by this point on the show hardly anyone can be bothered to treat these themes with any sincerity. There are a number of interesting ideas on display (creating artificial black holes, a labyrinth that's constantly shifting its layout), and some of the technical effects are quite good -- especially the collapse of the Anethan husk -- but they're not getting the treatment they deserve. Ah well, maybe things will improve in the latter half.
All that said, watching the co-pilot's death scene at the part two cliffhanger, and seeing as he falls that he's split his trousers, is terribly funny. The ultimate indignity.