They eventually decide that this must be some kind of alternate timeline. Haytham is reluctant to believe it at first- just because they've been unexpectedly forced into another century, there's no reason to believe anything out of the ordinary has happened. At some point, as he argues with Connor, Haytham makes the mistake of saying this last part out loud, and realizes how ridiculous his life has become.
And over the next few days, a number of details make it gradually more obvious that this world isn't the same as the one he remembers being forced out of ten months ago. The most obvious is George Washington- King Washington, as he's known here. His tyranny is nothing at all like the fledgling, struggling democracy Haytham knows should be here. And it's hard to imagine the country he saw in the twenty first century growing out of this nightmare.
There are other details, too. Ziio is dead- killed trying to stop the Mad King's rule. Connor never left his village, never became an assassin. And Haytham- honestly, he has no idea what sort of life he's lead in this world. He hasn't stumbled on anyone claiming to know him, and that makes him nervous. He keeps waiting for the other shoe to drop. Everyone else seems to be worse off here- he doesn't relish the idea of finding out his story.
Even with all this proof, even while his mind accepts this strange new world, he can't quite make himself believe in its existence. It's not until he meets back up with Connor in Boston that he's finally convinced. They've been separated for a few days while Connor travels with some of the people from his village in search of information. Only, 'finding information' somehow turns into 'killing Benedict Arnold', and the next Haytham hears of Connor is that he's being held in a Boston prison. Haytham goes after him, not sure if he wants to break him out or have it out with him for doing something so unbelievably rash without sufficient information.
He makes it into the prison, right up close to the cell where Connor is being held, without being noticed by anyone- including Connor. He's about to make himself known, he really is, when he sees something incredible.
He's looking straight at Connor when the man suddenly seems to… shimmer. There's a sound, almost like a snarling wolf, and Connor vanishes. Haytham can just barely see him when he switches to eagle vision, but there's something wrong with the blue shadow that marks his son's progress as he moves through the jail- sometimes it holds the shape of a man, other times, the shape of a wild animal.
Haytham corners him in a dark alcove, out of sight of the guards, and watches Connor reappear in front of him, a surprised expression on his face. "Dad," he says. "What are you doing here?"
"I came for you," Haytham says, then gestures at Connor. "Looks like you didn't need my help, though."
Connor shifts uncomfortably. "I was going to tell you everything, but things just started to spiral out of control, and-" he shrugs. "Here we are."
"What happened?" Haytham asks. "I saw what you just did."
"There's… a tea," Connor says. The hesitation in his voice is barely audible, and if Haytham hadn't spent so much time with him over the past few months, he doubts he would have noticed. "You drink it, and-"
"You turn into a wolf," Haytham finishes.
"You meet a spirit animal," Connor corrects. "And that animal guides you."
"What's the downside?" Haytham asks. "And don't tell me there isn't one. You wouldn't have gone running off on your own if it was safe." Even as he asks the question, his mind is still reeling. A tea? That ties the drinker to an animal spirit and gives him their powers?
This is the moment he truly starts to believe that they've somehow been sent to another universe.
"I don't know," Connor says. "I was told it wasn't safe, but…" he shrugs, uncomfortable. "They killed her."
"Ziio," Haytham says, in a voice so quiet he can barely hear it himself.
"Yea." Connor nods, and goes on. "Don't you see? I had to do this. They have to die. We need to get that apple away from Washington before he has a chance to do any more damage."
"I know," Haytham says. "I wouldn't expect anything less from you, not if they killed Ziio." He looks Connor straight in the eye. "But you shouldn't expect anything less from me, either."
"You want to help," Connor says, and for a second Haytham thinks he's going to argue. Instead, he only smiles faintly. "Thank you."
"Do you have a plan?" Haytham asks. "Preferably one that involves us getting out of here alive."
"I do," Connor says and holds up a vial. Haytham doesn't ask where he's been hiding it. "You're not going to like it."
"That's the tea," Haytham guesses.
"Enough for two," Connor says.
"You're right," Haytham says, and holds out his hand for the vial. "I don't like it. But I don't like anything in this world. If we're going to get the apple from Washington, if we want a prayer of getting back to our own world, we need to start playing by the rules of this place."
Connor nods, and without another word hands his father the tea.
They drink together, and the world dissolves around them.
-/-
Notice: I did change a few things from TOKW- namely, Ziio dies earlier, and the first part takes place over days instead of months. Also- obviously- Haytham drinks the tea with Connor. I haven't figured out how that's going to work out yet, but I'm pretty sure them dealing with it is going to end up as the meat of the TOKW plotline.
Yea, sorry, I'm rambling (It's late, I'm tired, shh).