Turn Left is the 11th episode of the fourth series of the revival Doctor Who. It stars David Tennant as the Tenth Doctor, Catherine Tate as companion Donna Noble and Billie Piper as Rose Tyler.
This is what is called a Doctor-lite episode, where the shooting schedule for the series dictates that The Doctor appear in only a few scenes in the episode. Instead, this episode focuses on Donna Noble.
The episode has a basic "It's a Wonderful Life" structure, where a temporal event (in the form of a very obviously plastic giant beetle) shows Donna what her life would be like if she had never took to traveling with The Doctor. The result is that Donna's absence would have killed the Doctor, most of his other companions, and leave the world a shattered, post-apocalyptic mess. But this cracked timeline brings in an even greater threat, because while Donna's life is spiraling downwards, a dimension-hopping Rose Tyler meets up with her, and tells her that her and the Doctor must save the world. By means of quasi-temporal paradox, she manages to bring about the end of her timeline, rejoining The Doctor just in time to carry Rose's message to The Doctor: that the world is ending. (In the forms of two words:"Bad Wolf").
And... there is a time when typing out the synopsis of a Doctor Who episode when I realize that any of the information I am going to convey is going to be impenetrable for someone who doesn't already know it and redundant for someone who does. Sorry about that last sentence, it kind of got away from me. And this episode is certainly one of them. But: in short, through a timey-wimey paradox, we are led to learn the lesson of It's A Wonderful Life: we all make a difference that we usually can not realize. And, of course, it sets up things for The End Of The World, again.