Ep 29. Season 6, Episode 10, "The Girl Who Waited"

In "The Girl Who Waited," the Doctor and his companions Amy and Rory arrive at a futuristic hospital where they discover a deadly plague has swept through the facility. After separating from the Doctor, Amy becomes trapped in a separate timeline where she lives for 36 years - alone and abandoned as a result of the TARDIS accidentally landing in the wrong timeframe. While Rory and the Doctor work frantically to rescue her from her exile -- which seems to be a scarier world than the one they had just left -- an older, hardened version of Amy comes to the forefront. The heartbreaking adventure tests the bounds of the Doctor's time-traveling abilities and the bonds of the trio's friendship.