Led by Queen Beryl, the Dark Kingdom waged war against the Moon Kingdom. After launching a destructive attack against the Moon Kingdom and massacring most of its people, including Princess Serenity, Queen Serenity, at the cost of her own life, used her powers to seal away the Dark Kingdom and send her people to Earth to be reincarnated in the present day.
However, the Dark Kingdom resurfaced in the present day with the aim of gathering enough energy to awaken its true ruler Queen Metalia and to take over the universe. In pursuit of this, Beryl and her top generals, the Four Heavenly Kings, hatched schemes to gather human energy and later sought out the Silver Crystal to fuel the Dark Kingdom's power.
After her first general Jadeite failed numerous times, Beryl did away with him and had him replaced by Nephrite. However, after Nephrite fell in love with Naru Osaka, he was executed by Zoisite, who saw his relationship with a human as a violation of the Dark Kingdom.
Zoisite and Kunzite then step up as Beryl's top lieutenants and head the Dark Kingdom's effort to gather the seven Rainbow Crystals, the shards of the Silver Crystal used to seal away the Seven Great Youma, the Dark Kingdom's most powerful monsters who had been reborn on Earth as six humans and a cat.) While they succeed in reawakening each monster, Sailor Moon is able to restore their true forms.
Eventually, once the Seven Crystals are found and united into the Silver Crystal, Mamoru and the Sailor Guardians all regained their memories of their past lives in the Silver Millennium and giving Sailor Moon ultimate power.
Zoisite was eventually killed by Queen Beryl disobeying her orders. The Dark Kingdom later captured Mamoru and brainwashed him to be their lieutenant. After the Senshi destroy Kunzite by putting their powers together as well, they head to the Dark Kingdom to fight Queen Beryl herself. After all of them are killed by the DD Girls except Sailor Moon, Beryl teleports Sailor Moon to the Dark Kingdom and has her face off against a brainwashed Prince Endymion. However, Sailor Moon manages to break Beryl's mind control on him, causing him to turn against Beryl and injure her. Queen Metalia then possesses Beryl, transforming her into Super Beryl, to fight the Sailor Guardian herself. However, Sailor Moon, upon becoming Princess Serenity, and supported by the spirits of the other Sailor Guardians, uses Moon Crystal Power to destroy Super Beryl and the Dark Kingdom once and for all.
Following the Dark Kingdom's downfall, the Sailor Guardians all return to life without the memories of the roles they once held, at least until the cats reawaken them in Sailor Moon R.