I'm going to make a completely random prediction, just because I think it'd be cool. Please disregard awkward dialogue, as I'm kind of sleepy and wrote this in one run-through.
Spoiler:
The TARDIS winds up, through sheer accident, inside the building where Hitler is staying. It's the day he was meant to die. If you remember, Hitler killed himself when he realized he was going to lose the war, rather than be captured.
The Doctor makes the mistake of telling them where they are.
Hitler is sitting in his study. There's a small kitchen attached.
Rory walks into the study. The Doctor and Amy open the door. They see Hitler face to face. They're too stunned at actually seeing the man to do anything. But not Rory. He walks up and punches him square in the face, knocking him out cold.
Rory: "Never thought I'd have the chance to do that."
They hear voices coming down the hall.
Amy: "What do we do?"
Doctor: "Quickly! Get in the kitchen.
Amy and Rory go to hide Hitler in the kitchen while the Doctor closes the door to the kitchen, then stands there as the two military officers come in.
"Where is the fuhrer?"
Doctor: "He went... thataway." Points down the hall. "Said he had a... a thing to do. You know, the thing with the thing?"
"Oh, right. I should've known."
They walk out and head down the hallway. Rory and Amy come out of the kitchen. Rory has a little bit of blood on him.
Doctor: "You didn't!"
Rory: "What?" Glances at the blood. "Oh, no... That was from punching him."
Doctor: "Where is he?"
Amy: "In the cupboard."
Cue intro.
Then they have a wacky episode with all kinds of hilarious hijinks and goofy chase scenes. But at the end, the Doctor, Amy, and Rory are watching through the cracks in the door as the two officers ask Hitler what he wants to do. Hitler describes a complex military strategy. The Doctor smiles for a second and says, "That's actually brilliant," then looks horrified. "He was supposed to die today. Hitler killed himself today. But he didn't because you clocked him on the head and he didn't have the time to think about it. You just might have cost the Allies the war."
Amy pinches Rory.
Amy: "Good going."
Rory: "Owww. Well, can't we just kill him?"
Doctor: "Then he'll just become a martyr, and make his cause stronger. The two of you. Back to the TARDIS. And stay there. Don't leave it for any reason. I'll be there in a few minutes."
The Doctor walks into Hitler's room. He holds up his sonic screwdriver and locks the (metal, to protect against bullets) door.
He then proceeds to explain to Hitler that he's seen the future, scare the everliving hell out of him, and convince him that being captured by the Allies would lead to a fate worse than death. Tells him about how Nazis are seen in the future. As nothing but monsters and genocidal maniacs who from dusk til dawn thought of nothing but killing off an entire race. Basically, convince him that suicide is the only option.
The Doctor walks out of the room, closing the door behind him. He pauses, and we hear a gunshot. He walks back to the TARDIS, looking down. Opens the door.
Amy and Rory both appear relieved he's alright.
Amy: "What happened?"
The Doctor: "History has been rectified."
Rory: "Well, that's good."
The Doctor, mumbling: "And I killed off two in an instant..."
Rory: "What?"
The Doctor, perking up: "Nothing. Where to next?"
Spoiler:
And then we all get sad as we realize the Doctor thinks he's worse than Hitler.