Abysmal.

It's a masterclass on how not to introduce a new Doctor - because for some reason, he's... asleep for most of it? This episode gives such little characterisation to the Tenth Doctor and that's because he's barely in it; it's baffling because there was a perfect set-up to explore this new incarnation. It's a literal invasion; let's see how this Doctor reacts to it, how he makes his plans (or wings it), how he interacts with his companions, how he does anything really? When he's finally awake, he's good, but it's too little, too late in the end.

Rose just kind of forgets all her character development from S1 and gives up immediately, even she spent the last episode saying she was taught to make a stand no matter what. The Doctor isn't dead, he hasn't left you; he's asleep - because he was saving your life from a situation much more dire than this, so what happened? Like the entirety of Parting is her coming round that she can make a stand on her own, the Doctor taught that and she's never gonna give up. AND THEN IN CHRISTMAS INVASION, she gives up because the Doctor isn't there. Brilliant. Utter character degeneration.

The pilot fish are really, really stupid. They blatantly know where the Doctor currently is because they send the tree to the flat (which they know the location of, somehow?), but they don't use the opportunity to kidnap him there? They waste time attacking Rose and Mickey? What's the point of the tree, just go in, kill Jackie and take him; Rose and Mickey are out. Their relationship with the Sycorax also feels really sloppy; their varying motivations make them feel superfluous like they were added solely for an extra set piece. The time might've been better spent on building the tension towards the Sycorax arrival.

Speaking of; the Sycorax are cool, ish, but translation of an unknown language just doesn't work like that and there's no sensible or reasonable explanation for the TARDIS being unable to translate just because the Doctor is asleep. But what exactly was their plan in the first place? The Doctor says they came because Earth had been drawing attention to itself, but what were they planning to do if they hadn't come across the space probe with the blood?

I'd be genuinely OK with calling this boring shite the worst post-regeneration episode AND Christmas Special to date.