Well, this is new.
Harry doesn't remember anything about aliens attacking Japan in the- is this the Edo period? He doesn't know. All his history lessons after the age of 11 were about Goblin wars. After that, he was rather put off the subject and never made time to do any self-study. But he's not here for a history lesson. He's got a job to do, because the Soul Society is apparently too busy to handle this.
The Master of Death picks his way across the battlefield, stepping around corpses and trying not to slip on the blood-soaked earth. Souls of humans and Amanto alike are collected on his way to the center of the fighting, where a figure in white is mowing down more of the beastlike aliens. Harry isn't actually here for the Shiro Yaksha. He's here for the bodies he's leaving behind and the souls of his comrades who are dying here on this battlefield.
Harry wants to get angry at Sakata Gintoki for the carnage and the resulting paperwork, but he can't. This is a man who has been pushed into a corner and is fighting to protect what he loves. Harry really can't get upset about that.
Gintoki and another comrade are pushed back-to-back, breathing heavily and watching the aliens surrounding them warily. Harry knows that neither of them will die today, and makes a note to come back to this dimension to visit. Sakata Gintoki has a brilliant silver soul, and Harry wants to know where his path in life leads.
I have a few of these crossovers with Harry casually strolling into different fandoms, but this one was finished first. Coming up with a vaguely Gintama appropriate title was unbelievably hard.
I really want to write some continuation one-shots. We'll see how it goes.