Touched by an Angel

In 2003, Rebecca Whitaker died in a road accident. Her husband Mark is still grieving. He receives a battered envelope, posted eight years earlier, containing a set of instructions with a simple message: 'You can save her.' As Mark is given the chance to save Rebecca, it's up to the Doctor, Amy and Rory to save the whole world. Because this time the Weeping Angels are using history itself as a weapon.