On Ferin II, a war rages. This isn't especially surprising; war is everywhere in this galaxy, so why should some out of the way planet on the Eastern Fringe be any different? This particular war was started because the planets Governor, Garius Merryn, developed a fondness for Tau service drones, force field technology and females (although to date no Tau female has returned this affection. Mostly they just find it weird). This pretty much worked out well for everyone, as Ferin grew wealthy and prosperous, and their improved technology meant that the Imperium was actually receiving more supplies than expected from the planet's farms and mines.
Unfortunately for everyone (except possibly the water caste diplomat that Garius had been pursuing), the Imperium is full of paranoid individuals, one of whom worked for the Adeptus Ministratum. He couldn't leave well enough alone, did some snooping and the next thing anyone knew 18 years had past and the Tau and Imperial Guard were wrecking the infrastructure of Ferin II in their pointless struggle for supremacy.
An atmospheric transport was zipping through the skies. It was carrying troops and supplies for the war effort, but due to the destination being on the ever-shifting front line, it was currently in enemy territory. Corporal Maria Note knew this, because her sergeant was taking great pleasure in trying to reassure her as she sat in the mess hall, presumably in his latest attempt to woo her.
"So you see," he boomed cheerfully, "you really don't need to be concerned about the Tau."
"I'm not," Maria muttered, ignored.
"I mean, yes, they could know we're here, but what could they do about it? We have a fighter escort, and point defence systems and everything. And anyway, they couldn't possibly scramble their interceptors in time to get us, we'll only be behind their lines for a short while. So don't worry! Everything is perfectly safe, and absolutely nothing could possibly go wrong!"
Maria froze for a moment at these words, then leapt out of her seat and sprinted for a nearby supply cupboard. Just as she reached it, the aircraft was rocked by a massive exsplosion that caused a table to slam into the sergeant. She heard more explosions, the lights went out, she braced herself against the walls of the enclosed space and then lost track of everything as the next few minutes were a confusing mix of tumbling, loud noises and swear words.
Eventually it stopped, and Maria gingerly explored her surroundings. The door was now above her, but after some shoving she managed to force it open. Clambering out she found herself in what was left of the mess hall. Bad news, most of it was on fire and littered with debris. Good news, there was a hole at the far side through which she could see daylight, and her sergeant was being cremated, which would save her a job. The cupboard was contained her entire squads kit, including her medi-pack and lasgun, so she grabbed the excess ammo and carefully picked her way outside.