This seems like it would be so easy, you can do it if you load it into iMovie, but if you want to do it really quickly, you have to use 5 year old software. mpegstreamclip does this, you just load the file and choose Extract/Video and it works.
Useful if you want to just look at the video clip and don’t want to import it into a real video editor.
Interestingly, the old QuickTime Player 7 also used to do this, but that feature was lost with Quicktime Player 10. You can still download it though.
Then there are commercial products like Video Editor MovieMator that try to upset you.