The season is divided into two arcs. 
The first arc shows Usagi and Co. having their memories restored from the first season. Their enemies are a pair of aliens, Ail and Ann, who are seeking human energy to restore their life tree. Mamoru had yet to recover his memories and he appeared as the Moonlight Knight rather than Tuxedo Kamen. This arc only lasted for 13 episodes. 
The second arc introduces Chibi-Usa, a little girl from the future who is searching for the Silver Crystal. The new enemies are the Black Moon Clan, comprising of the Ayakashi sisters (Cooan, Beruche, Karaberas, and Petz), Rubeus, Safir, Esmeraude, Prince Demando, and Wiseman. They too want the Silver Crystal so that they can take over the future. The Sailor Senshi's adventure continues...

Usagi Tsukino is an average student and crybaby klutz who constantly scores low on her tests. Unexpectedly, her humdrum life is turned upside down when she saves a cat with a crescent moon on its head from danger. The cat, named Luna, later reveals that their meeting was not an accident: Usagi is destined to become Sailor Moon, a planetary guardian with the power to protect the Earth. Given a special brooch that allows her to transform, she must use her new powers to save the city from evil energy-stealing monsters sent by the malevolent Queen Beryl of the Dark Kingdom. 
 
But getting accustomed to her powers and fighting villains are not the only things she has to worry about. She must find the lost princess of the Moon Kingdom, the other Sailor Guardians, and the Legendary Silver Crystal in order to save the planet from destruction. 
 
(Source: MAL Rewrite)

This season makes a turning point in the Sailor Moon story. The Sailor Senshi are confronted by a new enemy, the Death Busters. Rei had a premonition that this enemy would rule the world in an era called the Silence. To do this, the Death Busters need to find the three Talismans that would summon the Holy Grail. However, two mysterious Sailor Senshi, Sailor Uranus and Sailor Neptune, plan to find the Talismans before them... but without Sailor Moon's help.

Usagi Tsukino is a normal girl until she meets up with Luna, a talking cat, who tells her that she is Sailor Moon. As Sailor Moon, Usagi must fight evils and enforce justice, in the name of the Moon and the mysterious Moon Princess. She meets other girls destined to be Sailor Senshi (Sailor Scouts), and together, they fight the forces of evil!
 
(Source: Kodansha USA)
 
Notes: 
- The tankoubon release contains 52 Acts and 9 bonus chapters across 18 volumes; chapter/volume counts follow the tankoubon release. 
- The shinsouban release contains the original 52 Acts plus 8 new Acts across 12 volumes, but no bonus chapters as all 9 were released separately in <a href="https://anilist.co/manga/38552/">Sailor Moon Short Stories</a> alongside another bonus chapter that was previously released in an artbook. 
- The kanzenban release contains both the 60 Acts and the 10 bonus chapters from the short story shinsoubans across 10 volumes.

When Mamoru was little he gave a rose to a little boy named Fiore. Fiore promised that one day he would bring Mamoru lots of flowers. Now Fiore has come back to earth but his intentions are not merely to fulfil a promise...
 
(Source: Anime News Network)

Tsukino Usagi is a clumsy crybaby, but otherwise very cheerful 8th grade student. One day she meets a cat named Luna with a crescent moon on her head, and is transformed into the sailor-suited guardian of love and justice, Sailor Moon! As a chosen guardian of justice, Usagi has a mission … to find her comrades; to find the Phantom Silver Crystal; and protect the Princess. However, Queen Beryl of the Dark Kingdom also seeks the terrible power of the Silver Crystal, and sends her agents into the city where Usagi lives; triggering the start of a fierce battle.
 
Will Sailor Moon be able to find the other Sailor Guardians, and if she does, together can they find the Phantom Silver Crystal and protect the Princess?!

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