Possible Duplicate:
Can't the Doctor just land the TARDIS outside New York during 1938?
Spoilers:
At the end of the episode The Angels Take Manhattan both Amy and Rory get 'zapped' back into the past by the Weeping Angels, leaving the Doctor alone.
But since the Doctor is still in the present and with his TARDIS (unlike in a previous episode with the Angels), why can't he travel to the past to the point Amy and Rory get ported to and save them?