Sailor Moon Sailor Stars is split into two arcs; the first arc deals with an enemy they thought was dead: Nehelenia, who comes back with one intention - to destroy everything that Sailor Moon loves. After they defeat Nehelenia, a new threat comes from beyond the solar system, along with a new senshi, the Sailor Starlights. But determining whether they are friend or foe is harder than it looks. And on top of that, Mamoru has gone overseas to study and Seiya of the idol group the Three Lights is interested in Usagi.

On a training journey in the Bayankala Mountain Range in the Qinghai Province of China, Ranma Saotome and his father Genma fall into the cursed springs at Jusenkyo. When someone falls into a cursed spring, they take the physical form of whatever drowned there hundreds or thousands of years ago whenever they come into contact with cold water. The cursed will revert when exposed to hot water until their next cold water exposure. Genma fell into the Spring of the Drowned Panda while Ranma fell into the Spring of Drowned Girl.

Upon returning to Japan,the pair settle in the dojo of Genma's old friend Soun Tendo, a fellow practitioner of Musabetsu Kakutō Ryū or "Anything-Goes" martial arts which Genma passed on to Ranma. Genma and Soun agreed years ago that their children would marry and carry on the Tendo Dojo. Soun has three teenaged daughters: Kasumi, Nabiki and the hot-tempered, but helpful, martial arts practicing Akane. As Akane is Ranma's age she is appointed for bridal duty by her sisters. Although both initially refuse the engagement, they are generally treated as betrothed and end up helping or saving each other on numerous occasions. They are frequently found in each other's company and are constantly arguing in their trademark awkward love-hate manner that is a franchise focus.
Ranma goes to school with Akane at Furinkan High, where he meets his recurring opponent Tatewaki Kuno, the kendo team captain who is aggressively pursuing Akane, but who also falls in love with Ranma's female form without discovering his curse. Furinkan serves as a backdrop for more martial arts mayhem with the introduction of Ranma's regular rivals, the eternally lost Ryoga Hibiki, the nearsighted Mousse, and Ranma's perverted grandmaster Happosai. His prospective paramours include the martial arts rhythmic gymnastics champion Kodachi Kuno, the okonomiyaki vendor and his second fiancée and childhood friend Ukyo Kuonji, along with the Chinese Amazon Shampoo, supported by her great-grandmother Cologne. As the series progresses, the school becomes more eccentric with the return of the Hawaii-obsessed Principal Kuno and the placement of the power-leeching alternating child/adult Hinako Ninomiya as Ranma's English teacher.

Sara had forgiven Tarō Mitsuki for the mistakes he made in the past (her hate for him disappeared and her black hair reverted to its orange color). But since she became such a close friend to Gaito and she wanted to share his fate for everything he has done, she decided to entrust her orange pearl to Lucia and join Gaito in his demise. The Marine World may go back to normal, but it has one less Mermaid Princess.

Enter Seira, the new owner of the orange pearl. But she's not born yet; just a spirit who appears to Lucia in her dreams. In order for Seira to be born, her heart (心, kokoro) must merge with the orange pearl. But even before this happens, her heart is stolen by an angel named Michel. Lucia (together with Hanon and Rina) must retrieve pieces of her heart, even if she will face an almost unbeatable foe.

On the other hand, Kaito and Lucia love each other more, especially that Kaito has finally figured out that she is the mermaid who rescued him seven years before. But they are about to be separated once again when Kaito bounds for Hawaii to continue searching for the truth about his identity. But while in Hawaii, a surfing accident erases all of his memories. He is found and tended by a girl who has feelings for him. Lucia has to cope with this shock and attempt to let him remember at least some of his memories, especially of her.

Along with Lucia's romantic troubles, this season also highlights Hanon and Rina's love stories. Hanon rejects a persistent suitor, but finds newfound love with him after her fixation with her music teacher, Tarō Mitsuki, ends in disappointment. The story also covers Rina's first experience of love.

Many new villains and villainesses appear, including the return of some older characters. This season also features songs for the villains as well as newer ones for the mermaids.