Crystal Tokyo is a special city said to be 1000 years into the future. Ruled by King Endymion and Neo-Queen Serenity, it is a beautiful city with high crystal towers, peace, happiness and prosperity - a center for business and the arts.
Guarding the city are special warriors who are responsible for keeping the royal family safe and secure (quite possibly, the Sailor Scouts of the future). "Small Lady", also known as Rini in the real world, is the loving daughter of Endymion and Serenity, also the heir to the throne of Crystal Tokyo.
Going back into the past (which is our present), evil warriors form the Negamoon have targeted the city as their main point of attack and Rini as the future Queen of Crystal Tokyo, so their mission is to capture Rini in order that this event will never happen. It is up to the Sailor Scouts now to go with Rini to her home in the future and make sure that the Negamoon's plans are stopped.