So where is/was this backwards-time thingy?

This episode has got to be the best of the current Doc's, though once again I don't think Matt really contributed to it's greatness. He was darting about with the occasional idea that progressed the story but it was once again Amy that gave the emotional depth. It was also good to see a "family romp".

Amy and Hubby's leaving was the only logical conclusion to this short series, the subject had come up in at least two of the previous four episodes. It was their time, no pun intended, but it was still sad... On the other hand it will be good to see Doc and RiverSong in action as I think they work very well together.

Question: What is this "fixed point in time" that stops the Doc from going back in time to get Amy? In Blink the Tardis was sent back to the Doc and Blondie after the Angels had sent them back in time. So why can't the Doc or RiverSong go back?

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