Mami and Tomoki have always shared everything in life, they are identical twins, yet so different in character and personality: Mami tender and helpless, courted by boys to whom she doesn't never "no" for excessive kindness, Tomoki with a strong sense of duty and a great spirit of sacrifice, would do anything to see her sister happy. But comes a time, however, when one realizes there are things cannot be shared, especially when it is something extremely rare and unique .... just like an alien?!