In the closed and looming alleys of Oxford, in the cobbled side-streets of Cambridge, there is a door.

You have to know where to look. You have to be there on the right day of the week, wearing Oxford sub-fusc and a scarf from one of the Cambridge colleges, and one other garment from one other university, anywhere in the world. You must be alone.

You have to come at the right time, and in the right weather. In Cambridge it will be frosty, with the Urals wind low. The cobbles are treacherous, the Pembroke bowling green empty of even the most hardened of Fellows. Sometimes this will happen in summer, and then you will know it can only be the call of the Door. In Oxford there will be a thick mist reaching right up to the second floor of Waterstones. Somerville is disappeared into it, Keble a strange hallucination from someone who hasn't seen Selwyn for a while and is on an awful lot of drugs. This must be the case whichever Door you're aiming for, so it is vitally important to check the weather forcast for both cities, not just the one you are leaving from.

You find the Door. Perhaps it's somewhere near New College in Oxford, somewhere near Trinity in Cambridge. I don't really know. But now, you do.

You find the Door. It's not a thick oaken door, or an elaborately-carved mahogany door, or a scary metal safe-door, or even a saloon door (though there is one of those that takes you from Hitchin to Baldock, should you wish to make the journey). At first glance it looks like a mirror, standing all by itself.

And then you notice that the reflection isn't quite a reflection.

You walk through the gap, and there you are. In The Other Place. In The Fenland Poly. Crossed a hundred miles to find everything strangely, disorientatingly familiar.

There's only one problem: you can't go back the same way. It is estimated that in the last decade, something like 20 Oxford students and 30-40 Cambridge students have failed to turn up for important exams, due solely to the fact that they had gone through the Door, into the wrong city.

No one knows how long the Door has been there. There are no documents that mention it in either university, though a strange, obscure passage in "The Faerie Queene" has been cited as evidence by one or two peculiar Renaissance fellows.

Oh, but it is there. Go to both cities. Sniff around. Even though you had sworn it couldn't be possible, sitting comfortably at your computers in London or Newbury or Hitchin or Bedford or Edinburgh, once you're there, you'll know. And you'll vow that one day, you're going to find it.