All the Sailor Senshi, Princess Serenity, and Prince Darien were reborn in Tokyo without any memory of their existence on the Moon. The Silver Imperium Crystal was split into seven Rainbow Crystals and were born into seven human hosts on Earth.
In 1980, when Mamoru (Darien) was six-years-old, his family was in a car accident. His parents died and he lost his memory. While at the hospital, he met Fiore, an alien from another galaxy. Fiore provides Mamoru the comfort and friendship he needed but Fiore eventually had to leave Earth before Mamoru left the hospital because his body could not survive on Earth. Mamoru cried in his room when a little four-year-old girl, Usagi (Serena), walked in and gave Mamoru a rose to celebrate the birth of her baby brother. Mamoru gave his rose to Fiore as a parting gift. "Thank you," Fiore said as he began to disappear. "Someday, I'll bring you lots of flowers."
Since then, Mamoru had a recurring dream of a princess calling out to him to find the Imperium Silver Crystal. Mamoru, now nearly finishing high school, still had no idea of who he really was but he believed that dream was a clue.
Meanwhile, on a distant planet, an injured Makaiju (Doom Tree) escaped from a distant planet that was destroyed by war and anger and hoped that two of her children, Ail (Alan) and Ann, would find the energy of love, which would heal it.2
Nine-years-later, Professor Tomoe's gene manipulation experiment exploded, killing his wife and his eight-year-old daughter, Hotaru. "Do you want to save your daughter's life?" Pharoh 90 asked. Professor Tomoe cried, "I'll do anything to save her Hotaru!" A black star entered Professor Tomoe's right eye and he became possessed by Daimon Gerumatou. Hotaru became the human host of the daimon, Mistress 9.
In 1991, as Queen Serenity had warned, Queen Beryl and Metallia (Negaforce) somehow freed themselves from the seal and were wreaking havoc on the city of Tokyo, where everyone had been reborn. Queen Beryl commanded all her warriors to search for the Imperium Silver Crystal, which would give her immeasurable strength. Luna and Artemis separately began to seek after the Sailor Senshi and the Moon Princess.1
Artemis was the first to find a Sailor Senshi. He followed Minako (Mina) to London, England and revealed to her that she was in fact Sailor Venus. Sailor Venus took up the codename Sailor V and fought crime in London and eventually in Tokyo until the other three guardians and Princess Serenity was found.
Despite their moment in the hospital eleven years ago, Usagi hated Mamoru because he always teased her, especially about her hair. He called her "Odango Atama", meaning odango head. (Odango is a Japanese sweet dumpling.) "You are the person I hate most in the world," she told him once.
In March of 1992, Luna discovered that Usagi, an eighth grader, was Sailor Moon, the leader of the Senshi. Luna told her that the mission of the Sailor Senshi was to find the Imperium Silver Crystal. After it is found, the Moon Princess would be revealed and the Sailor Senshi would need to protect her, so that she can defeat the Dark Kingdom. Soon after, Luna also found Ami to be Sailor Mercury and Rei to be Sailor Mars. Jadeite was the first general of the Dark Kingdom that the Sailor Senshi fought and Queen Beryl killed him after too many failures. Nephrite then took his place when Makoto (Lita) was discovered to be Sailor Jupiter.
Usagi's friend, Naru (Molly), fell in love with Nephrite and helped him become good but Queen Beryl had him killed by Zoisite. Naru was of course heartbroken but Umino (Melvin) helped her get through it and they started dating. At this time, Artemis and Sailor Venus reunited with Luna, Sailor Moon, and the three other guardian Senshi. Together the Sailor Senshi fought the Dark Kingdom generals, Jadeite, Nephrite, Zoisite, and Kunzite (Malachite).
During this time, Mamoru as Tuxedo Mask fought along side Sailor Moon and assisted the Senshi in their battles. At first, Darien didn't even know he was Tuxedo Mask. He would automatically transform whenever Sailor Moon was in trouble. He eventually realized his alter ego and wanted to use it to find the princess that called out to him in his dreams, hoping that she could tell him who he was. Sailor Moon soon fell in love with Tuxedo Mask since he always showed up to rescue her from peril.
Jadeite was killed by Queen Beryl after too many failures. Zoisite killed Nephrite after he failed too many times as well and wanted to take his place. Finally, it was discovered the seven Rainbow Crystals were needed to find the Imperium Silver Crystal. By the time all seven were found, Tuxedo Mask had four and Zoisite had three. The Sailor Senshi began to suspect Tuxedo Mask was an enemy because he would not cooperate in handing over the crystals to them. But oddly enough, Tuxedo Mask gave Sailor Moon the Star Locket, a beautiful little music box.
In October, Zoisite proposed a dual with Tuxedo Mask, winner taking all the seven crystals. During this dual, Sailor Moon and Tuxedo Mask revealed their true identities by transforming in front of each other. Mamoru was fatally wounded by Zoisite and Usagi cried over him. The seven Rainbow Crystals merged with one of Usagi's tears and formed the Silver Imperium Crystal. Usagi them was transformed into the Moon Princess, Princess Serenity, and held the wounded Mamoru. She called "Endymion" and some faint memories of the past returned to her. She remembered they were lovers and that he went away to a war. Suddenly, Princess Serenity fainted.
The wounded Mamoru was then stolen by Kunzite and taken to Queen Beryl. Queen Beryl killed Zoisite for trying to kill Mamoru and then healed Mamoru's body but brainwashed him into joining the Dark Kingdom. Mamoru began fighting against Sailor Moon as Prince Endymion and it was extremely painful and emotional for Sailor Moon to fight him. She continued to believe that they would be able to bring his true self back.
It was now January of 1993 and the fateful battle with Queen Beryl was on the horizon. Queen Serenity appeared to the Sailor Senshi and whisked them away to the moon. She told them about what happened in the Moon Kingdom one thousand years ago. When the Sailor Senshi returned, Kunzite attacked them. Sailor Moon deflected one of his poisonous boomerangs back at him and he was killed. The Sailor Senshi traveled to D-point on the North Pole, Queen Beryl's hideout.
On the way there, they were attacked by Queen Beryl's five most powerful demons. The demons one-by-one starting with Sailor Jupiter, then Sailor Mercury, Sailor Venus, and finally Sailor Mars killed each Sailor Senshi. Sailor Moon became devastated and wanted to give up but the spirits of the dead Senshi told her to keep going. When Sailor Moon reached D-point, she found Queen Beryl with Prince Endymion together.
Prince Endymion attacked Sailor Moon but she fatally wounded him with her tiara attack. She then healed Prince Endymion by reminding him of their love with the Star Locket and he died. Sailor Moon transformed into Princess Serenity and faced Queen Beryl, who received a power-up from Metallia. Together with the life forces of the dead Senshi, Princess Serenity used the Crescent Moon Wand and defeated Queen Beryl. However, the power drained Princess Serenity of her life and her dying wish was that she would become a normal teenager again.
With the Dark Kingdom defeated and all the children of the Silver Millennium dead, the Imperium Silver Crystal granted Princess Serenity her wish. Time was turned back one year, before any of the Sailor Senshi discovered their identities, and time continued as if the Dark Kingdom never existed.
1 In Sailor Moon R episode 59, Ail and Ann are shown to be very young when Makaiju left their planet. Since they enrolled into Usagi's grade level at school, I believe Ail and Ann are the same age as Usagi so this would mean that Makaiju left its planet around the same time as Usagi and Mamoru's first meeting. 2 I believe that Queen Beryl was freed before Sailor V was discovered by Artemis because Queen Serenity said Artemis and Luna would awaken IF Queen Beryl was free again. If Queen Beryl was not free, Artemis would have no memory of the Moon Kingdom and would not have even started to look for Sailor Venus.