Things were very desperate at the conclusion of the previous episode, and they start even worse in the season finale episode. It begins with a computer read-out screen -- presumable on an alien spaceship -- giving a warning that Sol 3 was now off limits, and its natives facing extinction. This is all due to The Master and his mysterious henchmen, the "Toclafane". Six billion of the sphere-shaped Toclafane machines fell to Earth one day and The Master took over the world.

A year later, with half the human race dead and the survivors reduced to slavery, there is only one hope left.

Her name is Martha Jones.

There's very little more I can say without this becoming utterly spoilerific, so I will stick to generalizations.

In all, the plot was very intense and gripping. My fears about the origins of the Toclafane were realized, and I literally recoiled in horror at them, for they have traded too much to stay alive. (More on this after its US broadcast.) One of the themes that recurs throughout Doctor Who is summed up in the phrase, "Everything has its time, and everything ends," and we see in the Toclafane one of the possible consequences of fighting against an inevitable end.

Martha really comes into her own here, even more so than in "Human Nature / The Family of Blood", as the freedom-fighting heroine who saves the world. It's her that makes possible the moving moment when the remnants of humanity, united, empower The Doctor to stop The Master.

With history restored to its proper course, we learn a startling revelation about Jack, which probably means that we've seen the last of him in Doctor Who. Thank goodness for Torchwood, though! And Martha makes her farewell as the strong young woman we've always known her to be.

Yes, Freema Agyeman is leaving the series as a full-time cast member -- for now. She will appear in several episodes of Torchwood, and will also rejoin Doctor Who in the middle of the next season. But the Doctor will soon have a new companion in the TARDIS.

My nit picks: I felt the music was rather overblown in places. No mention of Martha's brother, Leo. Jack's farewell is awkward, though his revelation is startling enough to excuse that.

All in all, a satisfying conclusion to another season of Doctor Who.

Doctor Who airs on SciFi with the beginning of its third season this Friday, July 6, at 8pm EST.

It returns to BBC One with the special episode "The Voyage of the Damned" at Christmas.