Naoko Takeuchi's Sailor Moon looks simple on the surface, but is actually a complex tale of justice, romance, betrayal, and reincarnation. The story begins with Usagi Tsukino, a young school girl who has a chance encounter with a strange black cat with a crescent moon symbol on her forehead. Usagi later learns that the cat is actually a talking alien named Luna, who advised that she is the reincarnation of Sailor Moon , an alien warrior whose duty is to protect the Princess Serenity, the heir to the Silver Millenium kingdom, with her moon inspired powers. This story introduces us to her companions, the Sailor Scouts, each a reincarnation of a celestial alien warrior representing their now fallen planetary kingdoms and also tasked as Princess Serenity's guardians/friends:
As well as her love interest, Mamoru Chiba, aka Tuxedo Mask.
Through their quest to search for the elusive Princess Serenity, more background into their past lives. Through episodic story telling, foreshadowing, and investigation, its discovered that thousands of millenias ago, the Earth was in a temultious peace treaty with the Moon Kingdom, which is referred to as Silver Millennium. This peace fell apart when Princess Serenity entered into a forbidden romance with Earth's crown prince, Endymion. This caused a powerful sorceress named Beryl to start a revolt due to her own heartbreak over her unrequited love for Endymion, which was further corrupted by an evil entity named Queen Metaria. This development leads to the destruction of all life on Earth, the surrounding planets, and the Moon. Princess Serenity, along with her Guardians and Prince Endymion, are killed by the attacks. This leaves only one person left alive, the heartbroken Queen Serenity, mother to Princess Serenity and Ruler of Silver Millennium.
The queen, using the last of her power and her life, decides to transport everyone's departed souls into reincarnation cycles on Earth, including her daughter. She does this by using the Legendary Silver Crystal, a royal heirloom powerful enough to revive an entire planet with only dire consequences to the user. This opens up a large amount of lore, as the Silver Crystal becomes the primary goal of the Sailor Scouts, who Luna advises should be with the reincarnated Princess Serenity. Through further investigation, its discovered that, while Luna and, later her companion, Artemis, are able to recall many useful bits of information, they are unaware of a massive clue.
There was no Sailor Moon in the previous life.
This information is not realized until later in the first season, where Usagi transforms from Sailor Moon into her true form, Princess Serenity, due to Mamaro Chiba becoming severally injured and then kidnapped by a revived and vengeful Queen Beryl. It's then revealed how Usagi and Mamaru are the reincarnation of Princess Serenity and Prince Endymion and both now will have to defend Earth from intergalactic enemies while also preparing it for the age of Neo-Silver Millennium, an era where they will reassume their previous identities and will rule over the Earth for the rest of their eternal lives.
This story has many facets aside from these facts that also come into play. Themes in this show include LGBTQ+ representation on both the protaganist and antagonist levels, depression, personal boundaries regarding intamacy, and many other points while also being light hearted enough that fans of all ages can enjoy the francise. The show began in the 90s and still remains a progressive and immersive story that dazzles viewers, regardless if they are watching the 1992 original television show or the 2014 remake, Sailor Moon Crystal. It remains a trendsetter and icon in the magical girl/fantasy genre with many today still able to remember the iconic Sailor Moon theme song and poses.
Plus, the transformation scenes are just so pretty.