Shoutarou Hidari, a hard-boiled private investigator from the Narumi Detective Agency, lives for the sake of protecting the peace in the city of Fuuto alongside his inquisitive partner, Philip. One fine night, Shoutarou encounters a mysterious woman named Tokime, who is soon linked to several cases of homicide and theft. Shoutarou's investigation leads him to people using Gaia Memories—memory units similar to thumb drives holding vast amounts of data—to turn into formidable beasts called Dopants. While the majority of Gaia users were put down by the detective team in the past, a number of them still remain, posing a great threat to public safety. Once more, Shoutarou and Philip, Gaia users on the side of justice, must fight as Kamen Rider W—the renowned hero safeguarding Fuuto—to eradicate the nest of villainous Dopants, all while uncovering Tokime's true identity and confronting the long-forgotten past.

The story centers around three characters—Uesugi Kazuya, his twin older brother Tatsuya, and Asakura Minami. Kazuya is the darling of his town as he's talented, hardworking, and the ace pitcher for his middle school baseball team. Tatsuya is a hopeless slacker who's been living the life of giving up the spotlight to Kazuya, despite the fact that he may be more gifted than him. Minami is the beautiful childhood girlfriend and for all intents, sister from next door who treats both of them as equals. Society largely assumes Kazuya and Minami will become the perfect couple, including Tatsuya. Yet as time progresses, Tatsuya grows to realize that he's willing to sacrifice anything for the sake of his brother, except at the expense of giving up Minami to Kazuya. And thus the story is told of Tatsuya trying to prove himself over his established younger brother, how it affects the relationship between the three, and both brothers' attempts to make Minami's lifelong dreams come true.

As the Holy Empire rises to power, the neighboring lands begin to gradually fall under its control. The Empire's conquest eventually reaches the small island of Erin, home to the Gael tribe. There, a priestess named Riannon is kidnapped to be offered as a living sacrifice to the demon king Arawn, a malevolent being rumored to have caused untold destruction in the past. Riannon's brother, First Warrior Arthur, rescues her, when Arawn suddenly materializes before them as a handsome grey-haired man. Hiding his true identity and remaining enigmatic, Arawn pledges his power and leadership to the tribe's cause and joins Arthur, Riannon, and their merry band of friends—including a talented swordsman, an agile hunter, and a group of ecstatic pixies—as they fight back against the Empire, while uncovering the dark secrets of the land along the way.

Using his Almighty Farming Tool, Hiraku Machio continues transforming his village into an even livelier community as new residents from across the region find refuge there. As he tends the fields that first brought everyone together, Hiraku gradually takes on a greater role by guiding the village's growth not only as the chief but also as a father, ensuring that each day counts.