The premise of RahXephon is the fight between Humans, and pan-dimensional invaders known as the Mulians, short Mu. The Mu appear to be human; the only noticeable physical difference seems to be their blue blood.
In December 2012, two floating cities appear above Tokyo and Sendai. During a conflict with humans that escalates to nuclear war, in 2013, the Mu envelop the entire city of Tokyo and some outlying suburbs within a trans-dimensional sphere resembling Jupiter. The sphere is referred to as "Tokyo Jupiter". This "absolute barrier" is generally considered impassable, although it turns out that with enough power and technology it is possible to cross and even destroy it. The Mu control Tokyo; its citizens are completely cut off from the outside world and have been led to believe that the rest of the Earth has been destroyed. The barrier has a dilatory effect on time, with time inside Tokyo-Jupiter passing about one-sixth as fast as the outside time.
(From Wikipedia).
The series consist of twenty-six episodes and one movie titled, "Pluralitas Concertio." A manga series consisting of three books followed, but slightly tweaked for a younger audience.