This is not a bad episode of Doctor Who. It is however one of the most forgettable episodes of Doctor Who. There might be more forgettable episodes than this, but perhaps fittingly, I can't remember any off the top of my head.

The plot involves a spaceship crew illegally mining from the surface of a sun or something (who remembers), but the sun is alive and so possesses a crew member who stalks and kills the rest of the crew, whilst the ship is falling into the sun. It's not a terrible plot, but there's not much here beyond the initial premise. On top of that both the supporting cast and the monster are very unmemorable.

This episode does have a few things going for it though. Firstly the fact that it unfolds in real time, with the ship having just 42 minutes until it falls into the sun. This gives the episode some real pace and momentum, that it is able to maintain throughout it's run time. I also think there are a couple of genuinely good moments. Probably the main one being when Martha (and forgettable handsome crew dude) get stranded in an escape pod, floating away to their doom. The shot of the Doctor and Martha looking at each other as she slowly drifts away is probably the only truly memorable image in the episode.

So all in all there's little that's actively bad in this episode, and occasional moments where it's good. But it's mostly just ok. It moves fast enough to make sure you're never bored whilst watching it, but the moment the episode finishes it'll mostly slide immediately out of your mind.