I actually do think that complex plots are good for D&D mysteries, as they allow divination spells to be useful without being game-breaking. Here's the plot with which I began my current game:
The city of Manzikert has many noble families. One of them, the Pekkali, are very wealthy and powerful, headed by a brilliant old widower; another, the Goshraki, are impoverished and in a downward spiral, headed by a middle-aged dowager.
The dowager hires a team of sneaky criminals to infiltrate the Pekkali household, poison and charm the widower, and make him become infatuated with the dowager. Her plan is to woo and marry him, so that when he dies she'll inherit his household.
Unfortunately, a gnome who works in a local bath-house realizes that something is amiss with the widower (a regular patron), and, being a professional blackmailer, puts together enough evidence that he's able to confront the dowager and demand hush-money.
The dowager pays up, giving the gnome an expensive piece of jewelry that the widower had given her (a piece that had once belonged to his departed wife); she then tells her hired goons about the blackmail, and they murder the gnome.
The PCs are in the bathhouse when the murder occurs, and find the piece of jewelry on him, a piece that can be traced back to the Pekkali household....
The basic idea with this plot was to start them off with something small and inexplicable that they could trace through larger and larger circles until they realized that they were dealing with plots at the highest levels of power within the city. They were first-level, so divinations weren't available; even if they had been, however, it would be hard to ask the questions that would cut through the mystery. "Who murdered the gnome?" would direct them toward one of the goons -- or, if you want, toward someone that the goons themselves hired. "Why was the gnome murdered?" would tell them that he was murdered to keep his secrets quiet.
Hmm...a scenario like this would have a real problem with Speak with Dead, I'm realizing. YOu can mess with it, though, to minimize this problem: perhaps the goons, knowing that the spell is available, will incinerate his body after killing him.
Speaker in Dreams does work as a mystery, although IMO it requires a fair amount of work to make it coherent. I started a
thread about modifications to Speaker in Dreams; you can check here for one set of ideas on how to change it.
Daniel
Daniel