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 movies villain Rasen, the titular Oni King himself, is by far the darkest villain in the franchise. Even taking into account the movies 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.
- 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.
- 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
- 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.
- Strong Family Resemblance:
- Natsume resembles her paternal great-grandmother Lucy.
- Keisuke looks a lot like Nate did when he was his age.
- Teens Are Short: Touma and Natsume are teens but are a fair sum shorter than their parents.
- 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.