The Arisaka sisters (Hatsune and Kazuki) and the Kagura siblings (Yuuji and Marie) lived next door when they were smaller. So close that they could climb onto each other's verandas from the 2nd floor. They would always play together until one day the Kagura siblings had to move.
It's been 10 years and the Kagura siblings are moving back! Yuuji's turned into a pervert and Marie isn't the crybaby she used to be?!
(Source: Wikipedia - MB)