This film is the same. In this case, a Russian scientist has stolen the launch codes for their nuclear missiles, and a laptop that will enable him to launch them. Only Ethan Hunt (Tom Cruise) can save the day.
Once it's all set up, it's pretty much exactly the same as the 3 previous films, except that they've replaced Ving Rhames with Simon Pegg as the computer guy (which is more believable if you think about it -whenever I think of Rhames, I can't help but think of Pulp Fiction).
It's not a bad film, it's just if you seen one Mission: Impossible film, you've seen all the Mission: Impossible films