After regaining consciousness, I have barely a second to figure out what's going on before I collide head-first at astoundingly high speed with some sort of financial building.

The impact takes me about forty feet through concrete and two completely different floors of offices before I come to rest. My head is dazed. Error messages inform me that my jaw is rapidly healing itself after being hit astonishingly hard with something which was probably approximately fist-shaped. Somebody just punched me through a building?

A further half-second elapses as I take in where I am. Opposite me is the eleventh floor of King & Son Law, which puts me in Morton Financial - the business district of the new city centre. A quick architectural assessment suggests the Morton building should hold up despite the large amount of concrete and ironwork I ripped open during impact, and the bits of plaster still falling around me, and the many screaming though uninjured people.

Then I realise that my short-term memory has been totally wiped. Not by the impact - by whatever hit me. Whoever hit me. Long-term is - thankfully - intact though, so I dredge through it for relevant material. I'm sam512, a sam500-model android, primary function: bodyguarding. (Huh?) Sophisticated AI brain, basically indestructible bodywork, superhuman speed, strength and skill, top-of-the-line reasoning and combat abilities, and some experimental emotional plug-ins... Fifteen Laws overridden, I discover to my horror, by a black market hack that looks to be only hours old. The last thing I remember doing is filing away the day's experiences - short-term memory - at midnight on Monday 17th, which according to my system clock is now nearly sixty hours ago.

I have just permanently forgotten what I've been doing for the last two and a half days.

And I am in the middle of a fight with somebody.

That somebody: the humaniform robot, looking exactly like me down to the clothes he's "wearing" (actually they're part of the body, they serve to dissipate heat among other things), who just leapt onto the ledge where I burst into the building. Another sam500 model. A quick glance at his beacon and I have his registration: sam513. Right after me on the manufacturing line. I know him from before. Worked with him on a few jobs. Swapped data once or twice...

He's already tensing for a direct fist-first leap at me when I hold up my hand to stop him, "Wait, wait!"

"What?" he spits back at me angrily.

"You just knocked out my short-term, I have absolutely no idea why you're fighting me!" I scramble out of my hole and assume a defensive stance in case he still wants to fight, but that seems to have slowed him down. He pauses.

"Are you seriously suggesting that I have now also knocked out your short-term memory?"

"Also?"

"You did the exact same thing to me about thirty-eight seconds ago. I tried to talk to you but you kept coming. That last jaw shot was just self-defence."

"So - both of us now do not have the faintest idea why we're fighting each other?"

"Yeah."

"Or indeed how long the fight has been going on?"

"Well..."

513 motions to the "window" and I go and look out. Below us is, basically, a skyscraper canyon of chaos. Cars are overturned. Glass is still falling from a large number of completely destroyed windows. People are fleeing. Police cars are approaching, flashing red and blue. Street lights have been bent out of shape, some even uprooted. There's a small crater in the pavement directly below and opposite the window where we're standing - that was probably caused by the recoil from sam513's most recent, gigantic punch. Newton's third law.

I make an impressed noise. "At least a few minutes." A helicopter buzzes overhead, beaming shutdown commands at us in the microwave range. Neither of us are affected. "So you've also visited an illegal codeshop recently?" I ask, tapping my head.

"Apparently," he replies. "Maybe we should turn ourselves in."

"What? And get de-cycled by a factor of two? I don't think so. Look. Hacks like these aren't cheap or easy to get hold of. And a fight this intense - it was clearly intended to end in death for at least one of us and to blazes with the consequences for the other. There must be a motive, a really major one. How far can you remember?"

"...Two days."

"Ditto. And from before that there's nothing I can see which would lead us into conflict - I barely knew you."

"...External re-tasking?"

"Doubtful."

"Maybe we should go to your place, look for clues," he suggests.

I glance at the approaching cops. "Well, if we're going, we need to go now. Follow me." I jump out of the window.

After I roll upright from my slightly miscalculated landing, a polite warning message in my hindbrain informs me of my leg systems' design tolerances and how close I just came to exceeding them. I ignore it and get up. 513 craters behind me - he lands a little better. I begin to jog away, towards the nearest subway entrance. Maybe we can lose the cops underground.

Then a thought occurs, and brings me screeching to a halt. What if 513 is lying? I really have lost my memory, but what if he hasn't? Suppose he's the antagonist here. Or we both have equal reason to kill each other. Wouldn't that be the perfect way to catch m—

A message comes through from 513 - broadcast loud in every band I'm capable of receiving. "This is for my wife."

The last thing I see as I turn - so slowly, oh so slowly, why turn, you idiot? you should have ducked, there were a thousand other things you could have done, now you don't even have time for a crash-transmit - is a manhole cover, whirring towards me at neck height.