ABSOLUTELY WORST Doctor Who episode ever? "Let's Kill Hitler"
It is the Season 6.5 premiere, and has River Song (in the guise of Amy's childhood friend Mels) hijacking the TARDIS to bring it to 1938. They encounter tiny time travelers in a human robot who are there to torture Hitler for his crimes against humanity. Mels dies, and regenerates into River Song -- who then poisons the Doctor. Amy and Rory get trapped by the tiny humans in the robot, and are attacked by the robot's "antibodies". Amy manages to use the Doctor's sonic screwdriver to turn the robot's "antibodies" against the time-traveling crew, and River has a change of heart and saves Amy and Rory from the same. Amy shows River the kind of person she will become (good), and River changes to good, saves the Doctor by sacrificing her remaining 11 regenerations, and is taken into the future where she "belongs".
It's total junk. I can't begin to say how bad this episode is. I want to rinse my mouth with hydrogen peroxide every time I even pass by the DVD. If I see it again, I'll start kicking puppies and taking candy from children.