The main plot is a force field which descends around the town with no warning. It's tall enough that a plane flying above crashes right into it. In one of the most effective scenes, though, we see a single cow sheared in half by the force field descending like a slick swift invisible razor. The setup - so far - is that people outside the dome can't communicate with people inside the dome, except by holding up signs and the like. But since they can't see inside the dome, they have no idea where the heads and eyes that might see the signs are located.
In addition to what caused this - aliens, people from the future, a secret government project, take your pick - we have an ample number of percolating subplots inside the sheer teapot dome. Local councilman Big Jim Rennie - played welcomely by Dean Norris of Breaking Bad fame - is up to some kind of hanky panky involving propane gas, which may or may not in some way have triggered the dome. Plus, his son is a love-sick psycho, who accidentally knocks out then locks up in his family's fallout shelter the girl who jilted him just before the dome came down. There's a smart-taking, hard-ass reporter with long curly hair played by Rachel Lefevre - from the Twilight Saga and White House Down - who may have a heart of gold, and lots of other game and appealing characters.
At this point, Under the Dome bears some resemblances to Flashforward, which also debuted with a stunning storyline and a fine set of characters. To succeed, Under the Dome will need to keep its breathtaking pace and focus.