Montgomery Scott stares at the padd that's just been handed to him.

"How accurate are these results?"

"To three decimal places, sir. As requested."

He scans down screens of data and then grabs his own padd and calls up the file he needs. It takes a while to refresh on his screen and, when he glances up, he realises he is standing in the middle of an intent circle composed of half his engineering staff and three lab assistants from bio-sciences, including the ensign who has just delivered the results from the post-mortem.

"Do you lot no have work to do?"

As if repelled by static charge the group scatter with a mixture of mumbled apologies and insouciant "I was just on my way to..." body language.

"Not you, Philippe. Come here. Take a look at this."

His deputy stands over his shoulder, frowning at the screen.

"What am I looking at?"

"This is the spectral analysis of the dust from the warehouse. Look what happens when we superimpose the results over the same data from the Alpha 177 samples."

The screens merge in a ballet of mirror geometry. "It's just as Mister Spock said. Same mineral composition. Minor variations in concentration of the active elements." He then swipes downward.

The lieutenant's finger traces the new graphs.

"And what about these? I can see the spiking."

"Those are the nuclear magnetic spectroscopy results from the Alpha 177. They came later."

There had been no time then. Not with an imposter on the loose, a landing party facing an icy death and the captain... well, not himself. The Enterprise's loyal chief engineer pushes that memory aside with an inward shudder.

But later, in the quiet of the night, in the narrow spaces between dilithium crystal meltdown and yet another impossible demand from the bridge, he had kept returning to those samples. To the diagnostics from the transporter systems. It had become something of an obsession. For his transporters to go so catastrophically wrong… it nagged at him like toothache. Even though, he'd told himself, the chances of something like that happening again were infinitesimally small. And now he's staring at proof that the universe has once again cheerfully ignored the odds.

There's admiration in Philippe's voice as the truth dawns. "So you isolated the dynamic isotope. The rest is just so much dust."

"Two isotopes," Scott corrects. "It's the reaction between them that produced the dynamic. And I thought I'd figured out what happened in the pattern buffers. But then this..."

He highlights the results from the bio-lab's post mortem on the second padd and brings them up side by side with the data from four years ago. "These results are from the doctor's own logs - the captain's, no - the imposter's scan results. You can see the similarities, what's happening at a cellular level. But what bothers me are the differences."

"No, that can't be right." The lieutenant focuses on the screen as if it's written in ancient Greek. "Look at the DNA degradation in the latest sample. It doesn't make sense. The estimated date of death is only four days ago."

Scott's assessment of his new deputy climbs another notch. He'd taken a chance on Philippe Caron. Played a hunch that the man's chequered history, swerving from bio-medical research to engineering via a brief foray in astrometrics was evidence of an intellectual all rounder rather than an inability to focus. Applying a multi-disciplinary approach to engineering conundrums had already proved successful on a number of occasions. He's hoping this is shaping up to be another one.

"This is the third time I've sent them back to the lab. Same results every time." He scrolls back to a previous screen, barely registering the swish of doors opening. "I wish McCoy were here. He'd..."

"Be careful what you wish for, Commander."

The silhouette at the door is outlined in exhaustion, leaning on one hand as if the owner lacks the energy to step over the threshold. But there's no mistaking the fury in the voice that vibrates across engineering.

"What the hell, Scotty. They won't re-launch the shuttles - they say they're awaiting your orders."

"Doctor McCoy. We were just-" But the figure at the door isn't in a listening frame of mind.

"Goddamit, man. I've still got casualties on the surface. And Jim and Spock are down there somewhere. Giotto says their communicators are offline. What are you playing at?"

Scott tries again. "Doctor, come in an' sit down before you fall down."

"But the shuttles..."

"I dunna think we'll be needing the shuttles. If I'm right, we can have all those casualties up here in just a jiffy. But I need you to look at this."

With reluctance the doctor detaches himself from the door frame and hauls himself over to the proffered chair. He casts a blank gaze over the screens presented to him.

"Dammit Scotty, I'm a doctor not a mathematician. Give me some data I can get a handle on."

The chief engineer leans forward to highlight the screen of medical input but he's too slow. "Wait..." McCoy grabs the padd and holds it with both hands. "Are those the post-mortem results?"

"Yes, it's as you thought. Cellular degradation. And that dust was everywhere. But.."

"The DNA. This can't be right."

With a distinct sense of deja vu the engineer suppresses a sigh. "We've checked and checked again... There's no mistake."

There's real horror in the doctor's face as he raises his head from the screen. "My god, of course. I should have realised. The duplicates - once they took over..." He does a rapid calculation in his head." And this has been going on weeks. Perhaps longer. Scotty, we've got to find Jim and Spock. We've got to get them out there. And fast."

-oOo-

Back with our boys next, promise. But this chapter demanded to be written.