After the deaths of their parents, three siblings - oldest brother Rory, youngest brother Leo, and a middle sister named Ellen - are sent to live with their selfish, greedy Aunt Vera in her boarding house, The Rimini House.
Rimini Riddle contains examples of:
- And I Must Scream: After attempting to murder the three siblings for the money, the two young Victorian girls in the picture drag Mr. Commisky into the picture, trapping him there for all time. He later makes an appearance in Season 2 through the kid's computer screen briefly, where it's implied he's been trying to escape his fate but with no luck.
- Big Bad: Mr. Commisky for Season 1. Otto the stuffed Otter and the Mommos for the rest of the seasons.
- Child Hater: Aunt Vera.
- Conveniently an Orphan: The three protagonists live with their Aunt Vera after the deaths of their parents.
- Cut Short: The show was cancelled before it could wrap up the story.
- Eldritch Abomination: The Mommos.
- Hazardous Water: Even though Rimini House is on the beach, the kids aren't allowed in the water.
- Living Toy: Leo's beloved toy otter, Otto, who turns out to be a servant of child-eating beings called Mommos.
- Not-So-Imaginary Friend: When Leo gets in trouble, he blames Otto.
- Portal Picture: Vera owns a portrait painted by the kids' grandmother of two young Victorian girls on a beach. When the sister accidentally cracks the glass while cleaning, she finds herself inside the picture, where she meets the girls and a fisherman.
- Valentine's Day Episode: The longer of the two surviving clips is from this episode.