During our project on writing mystery stories, we watched these Scooby Doo episodes. They fit in well with our fairground/theme park topic and helped the children to 'shape' their story.
The children used the mystery short story rubric at Intel's website to encourage the children to think about the quality of their writing.
This website was used to help to structure their stories.
Some great ideas and planning sheets are available at this Scholastic site.
Hope you find this helpful.