From the official BBC Doctor Who website. This mini episode begins with the scene at the end of the Power of Three and then proceeds through the unfilmed scene that follows, presented as a storyboard with a voiceover by Rory.
Brian tells Amy and Rory that they should travel with the Doctor and save worlds everywhere...
It is sunset and Brian is alone in Amy and Rory's house. He is watering the plants. There is a ring at the doorbell. When Brian opens the door there is a stranger there. He hands Brian a letter. Brian is perplexed and says that he doesn't understand; this isn't his house. He asks how this man managed to find him there.
The Man, who speaks with an American accent, tells him that the letter will explain everything and that he will wait. He steps past Brian, into the house. Brian looks at the envelope which says "Dad." He takes it through to the lounge to read it.
The letter, written by Rory, tells him that if the calculations are correct it is a week since they left in the TARDIS and that they are not coming back. They are trapped in New York, fifty years before Rory was born. He says that the letter will tell how they lived and how, despite it all, they were happy. First, it introduces Anthony, the man in the hall. He was adopted by Amy and Rory in 1946 and is Brian's grandson: Anthony Brian Williams. Rory goes on to tell Brian what a wonderful father he was as well as saying he knows how odd it must feel to have a grandson who is older than him.
Brian steps out into the hall. Anthony holds out his hand in greeting but Brian, stunned, steps past it and embraces him instead.