In her everyday life, Yuuna Yuuki is a hero. As proof, she is in her middle school's Hero Club, where she does her best to help others and bring a smile to everyone's face. But Yuuna, always up to any task, is about to become an even bigger hero. Mysterious destructive forces called Vertexes begin threatening the world she loves, and the Hero Club is called upon by a strange phone app to save it. Along with her best friend Mimori Tougou, as well as sisters Fuu and Itsuki Inubouzaki, they must transform into magical girls in order to battle the Vertexes. In between studying and putting on shows for kids, Yuuna and the Hero Club must fight for the very existence of their world and face the harsh truths behind their own powers, all the while discovering what it truly means to be a hero.

Like the R Season, Sailor Stars is divided into two parts: The first part resolves some conflicts from the SuperS season, and sees the return of the Outer Senshi, Haruka, Michiru, Setsuna, and Hotaru (now reborn as a child). In the second part Usagi bids farewell to Mamoru, who is going to America to study. At the same time, the Three Lights apper, an idol trio consisting of three boys, Seiya, Taiki, and Yaten. The new enemy is Galaxia, a woman who desires to rule the entire galaxy by collecting the Star Seeds of humans. Three new Senshi appear, the Sailor Starlights, who have their own reasons to stop Galaxia, but without Sailor Moon's help.

A new Holy Grail War is emerging in Snowfield, Nevada, where mages from around the world summon Servants, heroic spirits drawn from myth and history, to fight on their behalf. However, this war seems to differ from previous ones. The organization behind it appears to have used data from the Fuyuki Grail War for their ritual, resulting in strange happenings. Unbeknownst to most participants, this budding conflict is actually a fake Grail War. There exists another set of seven true Servants who are summoned to fight in the genuine Holy Grail War. One of these Servants is a powerful Saber-class who saves the life of Ayaka Sajou, his apparent master, after being summoned. With hidden agendas and unpredictable forces at play, the line between the fake and true Grail War becomes unclear, turning Snowfield into a dangerous battlefield.

Coming from a renowned magus family, Ayaka Sajou is much expected to partake in the Holy Grail War. But ever since her father and sister's demise in the previous battle, she has grown gloomy and unwilling to involve herself in the matters that killed her loved ones. Soon, she finds herself in grave danger at the hands of a knight and ends up summoning her own Servant named Saber. With him by her side, Ayaka ultimately becomes an official participant in the war she initially wanted to avoid so much.