This may be a little long. Oh, and it's a bit setting-specific, I'll try to make it as generic as possible.
Basically, you're going to need a few background things for this:
1) Some sort of wizard's fair, seminar, or other such gathering in an urban environment.
2) The group hosting the affair doesn't particularly like sorcerers- they consider them untrained threats, hedge mages, posers, whatever.
3) At least one minor local wizardly noble, who will die, and one major local wizardly noble, who is behind much of the plot (and that the PCs may never connect things to).
I don't know much about the Dragonstart setting, but if you've got the whole galaxy to play with, there should be something that fits.
So: the PCs, for whatever reason, are in town for the wizard's fair. They could be wizards themselves, or buying items, or pulling security duty (good for a later plot point), whatever. An underground group of sorcerers starts making trouble- just rhetoric and sabotoge at first, starting with things such as posting anti-wizard handbills, graffitting or vandalizing seminar rooms, stealing exhibts, etc. This soon (a day or two) takes a violent turn- monsters are summoned into crowds, fireballs are lobbed at lectures, etc.
Presumably, the PCs begin tracking things down. They can get main leads: a bard (specializing in speech and rhetoric instead of music) who did most of the rabble-rousing handbills, and a fringe group of violent sorcerers who are inspired by his work, and are essentially using the bard, but taking things further than he had intended. Also, they will be occasionally aided (and watched over) by a local detective, or captain of the guard, or whatever.
Things with the bard are a bit ambiguous. His younger sister is married to the minor nobleman, and the bard has recently been abroad. Upon returning, he has been trying to get to see his sister, only to be told by her husband that she is ill and not seeing anyone. Actually, she has begun to develop sorcerous potential, and her wizard husband is unhappy with this, so he is keeping her locked up and helpless in order to avoid embarassment to himself. The bard has been dealing with the sorcerers trying to find someone to help liberate his sister, and did not expect them to begin attacking large crowds of people. The PCs can either help the bard or turn him in, as they see fit (IMC they left him to his own devices, he got some other people together, went to mansion for his sister, and killed the wizard in the process).
Now, the sorcerer gang also has some secrets. For a while, they were just a group of disenfranchised arcanists annoyed because they couldn't get in with the wizard's society. Now, they have the bard's rhetoric to inspire them, and they also have clandestine support in the form of the major wizardly noble. This noble (an ongoing villain IMC) is supplying them with their magical offenses in order to destabilize things locally, as part of whatever plot you feel like working in later. Depending on how the PCs track the group, they may or may not fight it out with them.
The noble is not dealing with the group directly- she is using the aforementioned guard captain (actually corrupt) as an intermediary. He is shielding the noble and helping the sorcerers get in and out of their targets. He's likely the highest the PCs will get in the investigation, as the noble will ensure that the captain disappears at the end of the festival (either he dies, or he's taken somewhere and hidden against divination magics). It's also the captain's job to ensure that the sorcerers go out with a bang in their final strike, so that they aren't around reveal potentially embarassing information.
When I ran this adventure, it worked like this: a wizard PCs tracked down the sorcerers and managed to charm them. The sorcerers told them their plans, and the PCs "agreed" to help, then went to the wizards and prepared an ambush to capture the sorcerers. Meanwhile, the guard captain has heard about this through his contacts, and doesn't want the sorcerers captured. So he arranges an ambush of his own- as the sorcerers are going through the tunnels to their "rendezvous" with the PCs (where the wizards will capture them), the guard captain and his men take them first. He knows their route, since he gave it to them. He makes sure to get a few city guards he can trust to do questionable work (this included a fighter PC when I ran) and they assassinate the sorcerers, who never show for the ambush that the PCs set up.
In the end, the PCs knew what the guard captain had done, and had a fair idea of the noble behind the plot but couldn't pin anything on them, and couldn't find the guard captain.
