Recap
The other Amy explains what little she knows, that the TARDIS interior and exterior have merged, creating a space loop. Further, the other TARDIS has drifted a few seconds into the future, so stepping into one TARDIS takes one back a few seconds into the past. The Doctor confirms that his Amy steps into the TARDIS right after slapping Rory. When Rory thinks about what might happen if they’re stuck with two Amy Ponds, and Amy slaps him, the Doctor sends her into the other TARDIS to maintain the chain of events. ..
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Episode Quotes
The Doctor: Okay, when does this Amy step inside the box? We need to maintain a timeline.
Amy-2: Ah, as soon as she slapped Rory.
Amy: Okay.
Rory: Huh. No, why do I get slapped?
The Doctor: Because we have to stick to the established chain of events. One mistake and the whole timeline could collapse. We'd end up with two Amy Ponds forever, and then what would you do?
(Rory thinks about it, Amy slaps him)
Amy: Do I really look like that?
Amy-2: Yeah. Yeah, you do.
Amy: Umm. I'd give you your driver's license.
Amy-2: I bet you would.
The Doctor: Ohhh, this is how it all ends. Pond flirting with herself. True love at last. Oh, sorry Rory.
Rory: Absolutely no problem at all.
The Doctor: Okay, we're back in normal flight. The TARDIS is no longer inside itself, the localized time field is no longer about to implode and rip a hole in all casualty. But just in case... Pond, put some trousers on.