Doctor Who: Steven Moffat's Timey-Wimiest Episodes

River Song's story arc is the most ambitious thing the show has ever attempted, and this sixth series finale is where it all comes to a head.

By this point we'd already learned that River was an assassin bred through a series of complex time travel events to manufacture a "Fixed Point in Time" - an unchangeable moment in which River is destined to kill the Doctor.

But River is not the sort to accept her fate easily, and uses her particular abilities to defy time itself, resulting in the catastrophic collapse of all time and space into a single instant: 5:02 pm, April 22, 2011, in which everything that has ever happened or ever will happen takes place all at once.

With all the build up that fans had had about the Silence, the Doctor's death, and River's identity, it might have been impossible to hit a conclusion that satisfied everyone. But like River herself, the story did not go out quietly - it embraced the complexities inherent in its premise and pushed its imagination to dizzying heights

Of course, eventually the Doctor was able to persuade River to set things right , mainly by revealing he'd already found a way to out-manoeuvre death... just without destroying all history in the process.