At last! This could well be the episode I've been waiting eight weeks for. In what has been the most lacklustre of the modern Doctor Who series, this corker from Paul Cornell has restored my faith in the good Doctor and proved beyond doubt that the more relaxed pace of the double episode lends itself much more easily to richer, more dramatic stories than a single 45 minutes ever can.

If you transplanted Randy Jackson from his seat on the American Idol judging panel to the Doctor Who critic's chair he might well have said: "Wow! NOW the series has really started! That was the bomb!"

And it was. It had something none of the previous episodes has really had in any measure: mystery. What was going on? The Doctor and Martha were being chased. That much was established in the first two seconds, but who was doing the chasing? And where, when and how were they going to hide?

The first two questions played out in the first segment: At a boys' school in 1913, where the Doctor has strangely forgotten who he is and Martha is employed as a maid (although she, at least, remembers who she really is). The casual racism and bullying of an Edwardian all-boys school was captured perfectly in a few short scenes. Tight, crisp screenwriting at its very best.

Gradually the back story is filled in, tantalising snippets being dripped into the storyline as the aliens - the ones doing the chasing - land and begin to animate first scarecrows and later humans. Sniffing like inveterate coke addicts they are, in fact, hunting the Doctor by smell. The "how" of his hiding is revealed in a flashback to the opening of the story, when he stepped into the chameleon arch to make himself fully human, and somehow stored his Time Lord essence in a fob watch which to begin with he gave to Martha for safekeeping, but which later somehow ended up on his mantelpiece only to be lifted by the light fingers of Tim Latimer - one of the younger boys at the school.

The Doctor's suppressed persona is coming out in his dreams, which he's been capturing in a notebook. It also oozes out when Latimer opens the watch, giving him glimpses of his own future mixed in with the Doctor's past. And whenever the watch is open, the aliens (or as we come to know them: The Family) are alerted to the Doctor's presence. Meanwhile he is oblivious to the danger: too busy falling in love with Matron while loyal, trusty (and now jilted before she even had a chance!) Martha discovers exactly what's going on and tries to warn him only to be dismissed from employment for her trouble.

Finally, Jenny is possessed by Mother-of-mine and tries to trick Martha into revealing the location of the Doctor. When Martha proves too smart for her, she pulls out an excellent shrimp-shaped green ray gun and attempts to kill her before she can warn the Doctor. The Family follow Martha to the dance hall where the Doctor is enjoying a swift twirl with Matron. In one of the best cliff-hangers in recent memory, they give him an ultimatum: chose who's to die. Matron or Martha?

Without question this episode is the best of the series so far. It had everything - mystery, suspense, scary monsters, great sets, fabulous costumes, excellent plot, dialogue and pace and that terrible cliffhanger. Fantastic. More like this, please!

Concludes next week with
Doctor Who: The Family of Blood. BBC One, Saturday 2 June, 7.10pm