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.
After stopping briefly to flirt with herself, Amy goes into the second TARDIS... and Amy and Rory enter the TARDIS. They explain that the Doctor sent them into the TARDIS, and the Doctor sends his Amy and Rory into the TARDIS. The Doctor has the two newcomers stay where they are and tries to figure out the precise lever he needs to prevent a temporal implosion. Amy wonders how he can’t know, and the Doctor says that he’s about to find out. A new Doctor enters the TARDIS and tells the original Doctor to pull the wibbly lever. The old Doctor then enters the TARDIS, which disappears.
Things are back to normal, but just in case, the Doctor tells Amy to put on some trousers.