Spoilers: They don’t.
Following a high-speed joyride in a field, Amy, Rory and Mels (their newfound, long-term, best friend they just met from way back) hook up with the Eleventh Doctor in the middle of a crop circle.
As the fuzz approaches, they hijack the TARDIS and take it to a safe place, namely, 1938’s Berlin, where Mels regenerates into the River we know and gives Doc the kiss of death.
There’s a tessellating robot piloted by tiny time-travelling space vigilantes, too, though, who immediately identify the war criminal in the room. And it ain’t the Hitler in the cupboard.
Now, on the cusp of death, The Doctor must rescue his friends, redeem his assassin and throw on a top hat and tails.