Four years after the final battle against the Dark Kingdom, the Sailor Guardians have set aside their transformation items and moved on with their lives. Usagi Tsukino and Mamoru Chiba are preparing for their wedding, but the joy is tempered by bickering over the elaborate ceremony, leading Usagi to briefly call off the engagement. Their domestic troubles are quickly overshadowed by a new threat. The villainous Mio Kuroki, thought to have been defeated, is resurrected by a mysterious purple light and declares herself Queen Mio, the new ruler of the Dark Kingdom. Establishing her base in an abandoned amusement park, she unleashes a horde of clown-like youma called Pierrots upon Tokyo to drain energy from the population.

The Sailor Guardians are nearly helpless, as the Silver Crystal was destroyed in the previous conflict, leaving them unable to transform. Rei Hino, Sailor Mars, is severely injured while confronting Mio in her civilian form and is hospitalized. When Usagi attempts to protect a young girl from the energy-draining clowns, she and Mamoru, still operating as Tuxedo Mask, are ambushed by the Shitennou—the four generals of the Dark Kingdom—and taken captive before Queen Mio. The young queen announces her intention to marry Mamoru, forcibly making him her king, while planning to drain Usagi’s life energy after the ceremony.

As Mamoru and Usagi are locked in a cell, their personal quarrels continue until Mamoru tricks their guards and forces Usagi to escape. The other Guardians—Ami, Makoto, and Minako—feel their friend’s distress and rush back to Tokyo from their separate lives abroad. Appearing from the Moon, Queen Serenity provides the Legendary Sword, which can temporarily restore the Guardians’ power as long as their hearts are united. With the spiritual aid of the bedridden Rei, the four Guardians together pull the sword from its resting place and transform into Sailor Mercury, Jupiter, and Venus to join the final battle.

Meanwhile, the Shitennou reveal that they were never truly loyal to Mio. They revive to help their former master, Endymion (Mamoru), fight Mio’s twin youma guardians, Sword and Shield. With the combined power of the Sailor Guardians and their allies, Queen Mio is ultimately defeated, transforming into a giant plant-like monster before being destroyed. The crisis resolved, Usagi and Mamoru reconcile and finally proceed with their wedding. The ceremony is attended by all the Guardians, their families, and even the spirits of the Shitennou, who appear to offer their congratulations before disappearing. The special ends with Makoto catching the bridal bouquet, hinting at her engagement to Motoki.