Anyone who's seen Sliding Doors is familiar with the premise. A seemingly insignificant decision, maybe even not a decision at all, but a missed opportunity - something small, something you'd never think of twice - and yet, it's a turning point. A point from which a startlingly different life path branches, and the consequences could be monumental.

So the case is with Donna Noble, who is thrust back in time by a dodgy fortune teller and a "time beetle" (the less said about the big silly plastic bug, the better); to a literal crossroads, where she turns right, instead of left, as she'd originally done. That decision takes her away from her potential temp job at H.C. Clements, away from Lance, away from the Racnoss... and away from the Doctor.

And things only go downhill from there....

So - all the questions from last week, answered. Mostly. Just who River Song actually is - or, more properly, will be - to the Doctor is still a mystery. But other than that, this episode was a neat tie-up of the loose ends from the first part of the story, with a little wibbly-wobbly timey-wimey business thrown in for good measure.

CAL turns out to be Charlotte Abigail Lux, former dying child, saved or damned (you decide) to a half-life as a living mind housed in the world's largest computer with every book ever written for company. Charlotte, bless, had saved all the people - literally - who were in The Library when the Vashta Nerada attacked, the only way she knew how....

This week on Doctor Who, we learn the oh-so-comforting fact that shadows can, in fact, MELT YOUR FLESH. So, everyone who's troubled by an irrational fear of the dark? Not so irrational! Wander into the wrong patch of darkness, and you could easily find yourself skeletonized in just a few excruciating nanoseconds! Nighty-night!

Unsettling as that sounds, this episode was not as flat-out scary as last season's "Blink" (writer Steven Moffat's previous offering). Still, it was effectively tense, and dark, and suspenseful, with a few bonus touches of humor and plenty of interesting questions to be answered next week. We're left wondering: Who is this mysterious little girl? Who is the even more mysterious River Song and how did she get her hands on the Doctor's screwdriver (we'd all like to know that one)? Who is Doctor Moon and what branch of child psychology involves telling the child that nightmares are real?...

Oh, how I loved this episode!!

It was a complete 1920's murder mystery package with a silly sci-fi twist. Not only the plot and characters, but the costumes, the music (*bows to Murray Gold*), the settings... even the characters' names were fitting: 'Professor Peach', indeed. The direction and staging and editing were fantastic too: all the details were there, right down to the spinning newspapers, the wavy flashback effects, the pointing, the gasping... even plucky Donna had her very own magnifying glass for clue-hunting. Not that she needed it to see the SIX-FOOT WASP that some person who hates me decided to insert as the monster of the hour; but it did come in handy in fighting him off, so hooray for that....

Can I just give this episode an A+ and be done with it?  Seriously. If you've been watching this show, and you like it, you will love this episode. And if it had ended five minutes earlier, it would have made a brilliant series finale. They could have ended things there, with everyone celebrating on Galactica, and we would have all breathed a sigh of relief, wiped the tears from our eyes, and closed the book on the story of these people we've come to love, safe in the knowledge that everyone got their happy ending....