In my opinion the first truly great episode in the Revival era and probably Christopher Eccleston's best performance as The Doctor.

His first scene with Dalek is an incredible piece of acting. The Doctor goes on quite the emotional journey in that scene. He begins with his usual Doctorly concern, when he thinks he's in the room with just another alien, and one who has been tortured. Then once he learns that the alien is in fact a Dalek his emotions quickly turn to abject fear as he begs to be let out. As he realises the Dalek is damaged and not a threat his fear changes to a gleeful fury, as the Doctor mocks and berates the incapacitated Dalek. There's a genuine sense of joy in the Doctor's anger when he gloats about being responsible for wiping out the Daleks in the Time War. His rage then subsides and turns to guilt and sadness as the Doctor thinks on the Time War, his part in it and his place after it as the only one left. Before finally the Doctor becomes vengeful and tries to murder a defenseless Dalek. We see a new side to the 9th Doctor in this scene. In fact we see a few new sides. This episode as a whole and this scene in particular do an incredible job at establishing how threatening a Dalek is, as well as showing us the toll of their shared history with the Doctor, both before and during the Time War. A truly great performance from Christopher Eccleston and some truly great writing from Robert Shearman (in what is criminally his only writing credit for the TV series).

In fact this entire episode as a reintroduction of the shows most iconic villains, is pretty much perfect. For anyone who considered the Daleks to be a joke, just a pepperpot who is defeated by a flight of stairs, this episode demonstrates that (when well written at least) they're a truly formidable and frankly terrifying adversary. A single, broken Dalek still brings fear to our hero. Once it's repaired itself, the same single Dalek will efficiently, methodically kill entire teams of elite commandos. The Doctor insists that should it escape the underground complex, it will proceed to wipe out the nearest cities. It is also presented as incredibly smart and can now 'Elevate' to handle those pesky stairs (although both of these things were also established in their previous televised appearance 'Remembrance of the Daleks').

Almost as terrifying as the Dalek in this episode is The Doctor. A man who is desperate, afraid and angry. He not only tries to murder the Dalek in their first meeting, but he is prepared to sacrifice Rose to stop the Dalek. He immediately regrets this decision and lashes out at his allies (although they're not the nicest, most likeable bunch to begin with), and then decides to grab a weapon to kill the Dalek himself. You always know things are bad when the Doctor has a gun.

At one point in the episode the Doctor explains why a Dalek would just go around killing people. He explains

"Because it honestly believes they should die. Human beings are different, and anything different is wrong. It's the ultimate in racial cleansing"

Then later when talking with the Dalek, the Doctor is mocking the Dalek for being the last, for having no-one to give it orders and for having no purpose. He tells the Dalek

"The Daleks have failed! Why don't you finish the job and make the Daleks extinct. Rid the Universe of your filth. Why don't you just die?"

to which the Dalek calmsly replies "You would make a good Dalek."

It's a brilliant exchange. Demonstrating the anger, guilt and vengeance he feels as the last survivor from the Time War risks the Doctor losing his humanity. This is brilliantly mirrored by the Dalek, who in order to regenerate had to absorb time energy from Rose, a time traveler. But, in doing so also absorbs some of Roses humanity, and is mutating into something less Dalek and more human. Being less than a pure Dalek is an untenable situation for a Dalek to be in, it couldn't even bring itself to kill Rose (or indeed some other humans). So it does the only thing it feels it has left, and it self-destructs. As the Dalek cannot live with becoming less Dalek, the Doctor feels shame at becoming 'more Dalek' (even pointing a gun at Rose who stands between him and the Dalek). He doesn't know what to do, but if Rose can give a Dalek a little humanity, she might just be able to restore the Doctor's humanity as well.

Just brilliant stuff.