Door four is another purely mechanical job. Ferno makes a few preliminary attempts to create a thermal lance with her bracelets but the door barely heats up, while the air in the highly enclosed corridor quickly becomes a furnace. She has no protective clothing. It's impossible to continue. Kinetic pulses from reconfigured bracelet adapters are equally ineffective. The giant machine reverberates with an ominous DOOM on each impact, rocking a little, but the door itself stays immobile. Reluctantly, Ferno opens up the touch-sensitive control screen and reads the problem on it. It's a very long and complex question which presents a very large hypothetical scenario and at first glance appears to have three or four possible solutions. An ordinary magical engineering exam would require her to write out the solution on paper, including all her working, but this time around she merely has to cast the spell for real using whatever equipment is on her person. Which is a substantial amount of equipment, counting the satchel full of Stuff, including laptop primed for advanced spot calculations, close to Batman-utility-belt levels, but not a full complement by a long shot. She has to improvise, with a magic circle drawn in Sharpie using her staff to plot the radius accurately. And it takes three or four attempts because several of the possible solutions are dead ends-- if read more carefully, the problem contains details which negate those solutions.

Every time she got stuck on her work in school, and even in university, and put her hand up and asked a question, the eternal response was always "Have you read the question?"

A cubic green lightning bolt and the door clicks and unlocks. Despite everything, including being completely cut off from communication with the outside world, Ferno cackles.

At the next door the computerised voice explains a very simple problem which was encountered very early in the days of magical engineering and is currently unsolved, and tells her she has three hours to solve it.

"That's the Hennering Conjecture. I can't do that. People have spent years trying to crack it. I can't do that."

"Sure you can."

"Okay. Once more. Who am I speaking to? Is this an artificial construct or a real person?"

"This is an artificial construct."

"I've figured out that this whole machine comes from the future."

"Correct."

"That's why it's arrived here, on this site out of the whole world, because this is where the time machine is eventually going to be built. I can't imagine how much energy it would take to project something this gigantic any distance backwards in time, but it would definitely take a lot of space and this is the prime location. And it hasn't been proven conclusively impossible, so I believe that more than I believe anything about teleportation or cloaking. And this whole series of challenges is basically a filter designed to let only me through, provided I'm smart enough. Or, as you claimed, kill me."

"The stakes are as follows:"

"I know what the stakes are. You don't have to bargain with me for that! You've shown me what happens if I don't come to the future but why kill me if I can't do it? Hand on heart, it's going to be sixty years before somebody as good as I am gets this far in the puzzles. This is some Indiana Jones crap. We'll nuke this thing to pieces if we have to and you won't have any return vehicle. I could nuke this thing right now. It'd kill me and this whole thing would be over."

The voice restarts with the basic principles of the Hennering Conjecture. Ferno is getting tired and her water bottle is empty. She discards it, and starts working.

There is a big blinking clock on the wall showing how much time is left. After a while she retreats to the far end of the corridor and faces away from it, sat cross-legged at her computer, pulling up resources from cache (no internet) and trying to crack the Conjecture. Yes, hundreds of people have tried to crack it. And they have failed. But she hasn't. Which means her only chance is if she knows something they don't. Her only chance is if there are results that she knows that nobody else has tried applying to it. So she pulls up her academic archive and looks for things which sunk into obscurity and notes she never developed far enough to publish. She actually moves into hard research mode and derives two unrelated nontrivial results right there in the maw of the trap. And she connects her work together. From past to present and a few inches into the future.

If the trap had arrived six months ago she would have died inside it. But she gets it. It's like light dawning and she doesn't even get a firm grip on the solution before trying it out. She looks up and around and speaks a few syllables. They are enough for the clock to pause at eleven minutes and one second remaining. But the door doesn't open. She hurriedly fishes out a jar of incredibly expensive metallic dust and pour half of it, delicately, in a very small circle on the ground. Then she concentrates and says the same syllables again. And again, in a few more combinations. And one of the combinations is the right one.

"James Hennering's going to enjoy that one," she says, collecting all her equipment and hurrying through. Apparently, if time runs out, poison gas will be released.

The next room is huge and circular and it is instantly clear that it is encircled by a heavy cadmium/tungsten/iron Montauk Draining Ring, which is a charging device whose major function is to drain magical potential out of the environment it encloses. It doesn't suppress magical discharges so much as slurp them up and store them, like a vacuum cleaner. In other words, actual useful magic here is impossible. This is a significant point because there's a robot in the centre of the room and it is unfolding like a man-sized black feathery scorpion and now arrowing forwards like a flechette--

During the whole two seconds at her disposal, eighteen hundred milliseconds of which are spent watching the robot unfold and only two hundred milliseconds of which cover the robot's movement towards her, Laura Ferno remembers with a start that her grandmother, the late, great, Rachel Ferno, taught her everything she needed to know to get through every door up to the last one. And Granny struggled towards the end of her life with a problem equivalent to the Hennering Conjecture but was hospitalised and then institutionalised towards the end of her work and it was never completed. Granny died years and years ago and the loss almost killed her but she learned to rely on herself and the things which her Granny taught her which would never die. And Laura Ferno distinctly remembers never being taught non-magical combat. Or counter-robotic combat. Or any kind of combat.

And Laura Ferno remembers that behind the fun and the adrenaline-driven exhiliration of success and discovery under intense pressure, behind the bizarre Crystal Maze traps and the maniacal, implacable controlling computer system, there is a time machine.

She drops her satchel and swings her staff down. With or without magic, it's a two-metre rod of metal with weight behind it. Admittedly, it is only made of aluminium, which she chose for the lighter, more portable weight, and because she never expected to do physical combat with a machine. Steel would have been a better choice.

The robot dodges the swipe and swipes at her itself, with a fist. A fist? It moves too fast to completely dodge. Ferno's evasive manoeuvre is poorly-judged and she loses her footing and lands on her tailbone. BUMP. She instinctively kicks for the buzzing, black feathery thing's groin (it's humanoid) and this inexplicably holds it up. Then she tries another swipe with the staff, which it blocks. And then, as fast as a thunderbolt, she's flat on her back and there is a laser being shone into her eyeball. And a knife-tip behind it. For both eyeballs. Two knife-tips, two lasers. Two laser-guided knives, stopped a millimetre above her eyeballs. And the robot is poised behind it, holding the knives. If it could move that fast and with that much precision, there was no point in the foreplay. It could have killed her in an eyeblink. Through an eyeblink. "Holy shit," she says.

"You're dead," says the computer. "Twice."

And then the world goes black, because it's past midnight at her destination.

*

She falls for a second or two, falling ten or fifteen metres, and lands in a big duvet-like gobbet of what she can't see in the dark but which looks, when illuminated, like greenish-black snot. The stuff evaporates in a matter of seconds leaving her lying flat on her back, surrounded by her gear, in field of sleeping sheep.

If you had to find out what time and date it was, what would you do? She fumbles around for her satchel, and pulls out a torch. Then she locates her phone in one of the pockets. No 3G signal, but there is basic cellular connectivity, and because she's out in the open, there's GPS. Not too far in the past, thank God. A few minutes' hackery and she knows what time the GPS satellites think it is, and they're accurate to nanoseconds, which is good enough.

The speaking clock, idiot, says her brain after she has finished congratulating herself on this clever piece of improvisational time-determination.

A nested pair of time machines, Ferno thinks. Like Matrioshka dolls. A city-sized structure to send the first machine back. And the first machine was big enough to contain a second machine which was itself big enough to send back me and the cushion. Back to now.

Laura Ferno's phone starts ringing and she answers it.

"Yes."

"Can I speak to Rachel Ferno, please?"

"I... this... this is she."

"You're the mother of Daniel Ferno, is that correct?"

"Yes."

"I'm sorry to be the one to tell you this. There's been a car accident. Mr. Ferno is dead. It was just a few hours ago. You were listed as his next of kin."

Laura pauses. She waits for long enough to give the impression that she is surprised and shocked. Then, she tries to think what she is supposed to ask next. "What about his wife?"

"I'm afraid she also died in the crash."

One. Two. Three. "And his baby, what about little Laura?"

"Laura is fine. But she has nobody to take care of her, right now."

*

Granny never taught me to fight robots. I relied my whole life on what she taught me. I was never vulnerable as long as I remembered what Granny taught me! And it was all me! Call that 'self-reliance'?

Because she never got past the previous door. She never fought the robot. She "died" at the Hennering Conjecture.

Well, that'll change.