Yes, you're right about よ
It's like "hey", as you imagined.
Everyone knows it's A movie, but the tag-line is rather saying "Hey Japan, (pay attention,) THIS is THE movie".
In other words, the ad is saying "you don't know what movies actually are; watch this. THIS, is the movie".
The particle が is not only a subject marker, but it also works as a emphasising が. This means that the word right before が must be emphasised.
これが映画だ means "this is what we call movie" or "it's this that we say movie".
Hope this helps!