This whole arc is amazing and this episode starts it off well. We get to see Jack return and catch up with the Doctor while the Doctor tries to help out the last remnants of humanity at the end of the universe.

The atmosphere of this episode is just brilliant even before the twist at the end. The end of the universe is dark and brutal and scary and humanity is still hanging on. Something about the constant mentions of "oh there used to be (very common thing) thousands of years ago" just drives home how desperate the situation is (and it doesn't work out as we see in the next episode)

The reveal of the Master was fantastically done as well. You can even see how he is unlike the Doctor just in how Yana behaves vs John Smith. Having seen the 3rd Doctor's run now adds to it for me as well