The letter had read like this:
Hey Sirius. I’m Sirius, too. You might have noticed the resemblance. Granted, a Polyjuice potion could do that so I’ll list off some facts in no certain order…
His eyes had bulged when he read some of that, and the shock began to settle.
Believe me now? Please do. I’m giving you the opportunity of a lifetime and that is to have your lifetime.
There’s this thing called a Nexus where world’s collide. Imagine our fates played out in an infinite possible ways. Still don’t get it? Imagine a mirror reflecting into a mirror. That should give you some idea. It’s a bit hard to get your had around. The Portkey I provided along with my version of the bike will take you to one that has a ‘no-violence’ shield, so don’t get too rowdy or you get kicked out and we’re all up Shit Creek without a paddle.
Anyway, I stumbled into this Nexus thingy a while back and I found out that what’s just happened to you, happens to me, and I don’t intend that on happening. I met a bunch of nice people, even people from our future, and one day a baby Harry popped up. Your Harry. He’s orphaned and I really can’t look after him anymore without fucking up the future of your entire world.
This is where you come in. You’re his godfather. You have a responsibility. If he’s left with the Durseley’s, he lives an utterly shitty existence until going to Hogwarts. Neither of us wants that. And if you’re thinking ‘Oooh, I’ll just rot away and Azkaban’ shut the fuck up and be responsible for once. We’d do anything for our friends. This is James and Lily’s son. Granted he’s staying with her snotty bitch of a sister for now but you’ll remedy that soon.
On top of all this, Peter is alive. He’s changed into a rat and is hiding out at the Weasley’s as their son’s pet. No, don’t go off and kill him you silly git. He’s a bastard but Harry is the important one here. If you go to the Nexus, my friends will help you to capture him and you can hand him in or get someone from your timeline who’d believe you to hand him in. Or maybe if you give the Invocation Field a good kick, you can get him. Then you come back, switch places and they release you on time. Because I’ll be fucked if I’m rotting in Azkaban for the rest of my days.
Unfortunately, because of the spell Lily did before she died (i.e. giving up her life to save her son, etc, etc.) he has to live with blood relatives. Fortunately, you’ll be free and breathing down their necks and thus make sure they’re not giving him such a hard time.
Do it for James, Sirius, if you're in doubt about it at all. For James, Lily and Harry.
And so, when a familiar black dog turned up the next again day, he hadn't wasted time in changing to his Animagi form and switching places. He stared in awe as the other him took his place and gave him a sharp nod. With that, he'd ran away with instructions in his mouth as to where to find the bike, a Portkey and a dozen bottles of Polyjuice potion.