Quick Question.

For context: I started and am at Unova. 4 gyms down. My friends are in Kanto, and they are forcing me to join them. And I chose Unova because I haven't played Black/White before.

What happens to my main story progression if I left the region? Does it continue whenever I return? also, if I go to Kanto, can I return to Unova whenever I want to? or I need to be 4 gyms down there as well? and can I randomly switch between the stories as I please?