There are countless mysteries throughout history, so this should be a fairly easy aspect of mystery to incorporate into your themed lesson plan. For instance, you could discuss classic historical whodunits, focusing on the mysterious deaths of famous figures like Genghis Khan, John Wilkes Booth or Queen Elizabeth I. You can set the scene for students by talking about the historical period, and then have them investigate different theories surrounding the person's death. Ask the students to create their own personal theories for a creative writing exercise, or have them act out their ideas in some group skits.
Incorporate mystery madness into your science lessons. There are a variety of opportunities to explore the mystery of science, so try to find an angle that fits in with the material you're currently teaching. A basic way to work the theme into your lessons would be to talk about how the scientific method is used to help solve scientific mysteries; you could discuss theoretical disputes in the scientific community regarding a concept you've covered in class. You could also introduce your students to forensic science used to solve mysteries at crime scenes for an exciting study that is sure to grab their attention.
Study the genres of mystery and suspense in class. Highlight the characteristics of these kinds of stories, as well as famous authors and writing styles. Give the kids some hands-on experience with writing mysteries by having them pen their own original stories. These could be assigned for an outside-school project, or you could keep the mysteries short and simple with a 15-minute writing exercise at the beginning of class. Have the students read aloud what they've written, and require them to read in a mysterious or suspenseful tone during all of the parts that call for heightened tension.
Read some classic mystery novels in class, such as the Sherlock Holmes series, and then as a completion reward, have a movie marathon of the cinematic versions of the novels. If class time is too short to watch the entire movie, use the end of class bell as an opportunity to heighten the suspense, leaving the students hanging until next time. For refreshments, offer treats with mystery flavors, challenging the students to identify what they are eating using clues and deductive reasoning; if you decide to do this activity, double-check to be sure there are no food allergy risks.