The story starts thirty years after the anime. As Nate grew up he found he could no longer see yo-kai. His yo-kai watch has since been buried between space and time. Everything was peaceful until thirty years afterwards a comet appeared in the sky. An evil yo-kai virus has spread that causes the people of Springdale to become malicious. Luckily, a few teenagers have been chosen as the owners of the newest yo-kai watches.
Yo-kai Watch: Shadowside - The Return of the Oni King boasts a darker, more serious tone and a different art-style from its predecessors, while still keeping Yo-Kai Watch's signature humour. Yo-Kai Watch 4 features the characters from the film.
The film has a spinoff anime in Yo-kai Watch: Shadowside.
This film provides examples of:
- Alliterative Family: Keita (Nate) has a son named Keisuke. Keisuke's great-grandfather was named Keizou.
- Alliterative Name: Akinori Arihoshi.
- Art Evolution:
- The art-style is less cartoony than previous installments. Most noticeable are the yo-kai. For example, Komasan looks like a genuine dog instead of a Cartoon Creature.
- Inverted with the GeGeGeNoKitaro characters, who use their Showa era anime character models and colors rather then the style of the modern adaptations.
- Artifact of Doom: The Kigan Gear, a demonic bracer that allows it's bearer to control anyone who's being inspirited by the Onimaro. Once Touma's been purged of his Darkness, it turns into the Yo-kai Watch Ogre.
- Advertised Extra : Blizzaria, Blazion, Papa Bolt, Hungramps, and Azure Dragon only show up for less than a MINUTE. Justified for Hungramps as he has a Story-Breaker Power. Kyuubi gets hit hardest though. See The Worf Effect below
- Big Damn Movie: A lot more is at stake than in the main anime.
- Bloodless Carnage: Touma gets stabbed but doesn't bleed. He was, however, stabbed with a red object, which gives off the illusion of blood.
- Childhood Friend Romance: It turns out Nate actually did pair up with Katie. Thirty years later and the two are married with kids.
- Crossover: With GeGeGe no Kitarō.
- Darker and Edgier: The film features a less cartoony art style than the original anime and is a lot darker than the usual Yo-kai Watch fare. The protagonists are also older than Nate. Yo-kai are shown to have two types of forms: a light-side and a shadow-side. Nevertheless, there is still a good amount of slapstick.
- Disney Death:
- Natsume is assumed dead at one point. It turns out that that she was saved by a dragon yokai that she unknowingly helped years prior.
- Touma actually dies in a Heroic Sacrifice, but he ends up getting revived a minute later.
- Divine Race Lift: It's lampshaded that Enma looks nothing like his traditional design.
- Expy: Akinori looks a lot like Bear.
- Fat Bastard: Rasen is a largely overweight oni king, and is the Knight of Cerebus of the movie & the Yo-Kai Watch franchise. When he's De-Armored, he loses a few pounds.
- Friendless Background: Touma dislikes people and avoids making friends.
- Hate Plague: The virus makes people into evil youkai.
- Japanese Delinquents: Touma is shown getting beat up by some delinquent boys. They quickly become Asshole Victims.
- Kid Hero: The protagonists are secondary schoolers. They're three years older than Nate in the first anime.
- Kid Hero All Grown-Up: The film takes place thirty years after the anime. It features a new trio of kids obtaining yokai watches. Nate has since grown up and lost his ability to see yokai.
- Knight of Cerebus: The movie’s villain Rasen, the titular Oni King himself, is by far the darkest villain in the franchise. Even taking into account the movie’s Darker and Edgier tone, Rasen is a terrifying and sadistic monster whom the heroes are only barely able to beat.
- Next Sunday A.D.: The film takes place in the 2040s but it looks identical to the 2010s.
- Not-So-Forgotten Birthday: Deconstructed. Turns out that Natsume and her friends didn't forget his birthday and planned to surprise him by coming to his home to celebrate. However, Touma never opened the door and pretended that he wasn't there, believing it was the mailman, thus tragically missing his chance at not being alone.
- One-Person Birthday Party: Touma Tsukinami spent many birthdays alone in his youth. He could have avoided this if he opened the door of his home to reveal that Natsume and her friends were waiting at the entrance of his home to visit him, but Touma tragically mistook their hard insistent knocks for the mailman and pretended to not be at home, thus missing his chance to not being alone.
- Retcon: Once again it changes how long the Yo-kai Watches have been used and when the first were made, now dating back to ancient times.
- Pilot Movie: This movie serves as the Pilot for Yo-kai Watch: Shadowside, serving as a way on how the Youkai Detective Agency began in the first place.
- Short Teens, Tall Adults: Touma and Natsume are teens but are a fair sum shorter than their parents.
- Spin-Offspring: The film stars Nate and Katie's children, more specifically their daughter as their son quickly succumbs to the oni virus.
- Small Role, Big Impact: Despite only being in the movie for a few seconds Azure Dragon and his Kindred Yo-kai are able to destroy Rasen's armor, allowing the heroes to finish the job
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Spiritual Successor: The film resembles GeGeGe no Kitarō even more now thanks to its darker tone and less stylized yo-kai. It also helps that the film itself is a crossover with Kitaro. - The Stinger: after some months pass the Oni Virus Incident, Natsume, Touma and Akinori decides to start off the Youkai Detective Agnecy after knowing that there's other people with cases related to Youkai Activity. Meanwhile, on a dark castle above the sky, Shutendouji and Doketsu are looking for "the Princess" after their plan to set the Oni King to find her failed.
- Strong Family Resemblance:
- Natsume resembles her paternal great-grandmother Lucy.
- Keisuke looks a lot like Nate did when he was his age.
- The Worf Effect: Kyuubi gets hit with this hard. Only restraining Touma for a few seconds before being punched into a wall and electrocuted.
- When You Coming Home, Dad?: Touma's parents are often too busy with work to be with him.
