Great action flick, mediocre plot(this review contains spoilers)
An anarchist wants to contaminate the Siashen Glacier at the base of the Nubra River valley in Kashmir with plutonium in order to "end the old order" by starving 1/3 of the world's population, because, as his manifesto goes, "the greater the suffering, the greater the peace". Not a well thought out thesis really, but Ethan and the IMF team's attempts to thwart it makes for a good action movie.
There are some excellent car/motorbike chase scenes around some famous places in Paris - the Arc de Triomphe roundabout, etc. And some decent free-running in London. I guess they used CG to do the car/motorbike chase scenes in Paris, since I doubt they could have closed off such busy places in the middle of the day to film the movie. But it was very well done - certainly fooled me.