Wellstone City is not new to smuggling. The big crime syndicates all have established operations, and there are plenty of small-time crooks who bring in (or ship out) various shady goods.
However, a massive shipment of military-grade arms was just intercepted by an unknown party. The intended recipients, the Black Dragons, have millions tied up in this shipment and are keen to get it back and kill those responsible.
That’s where the PCs come in. The Black Dragons appear to be under siege, with all of their operations being hit by unknown assailants. They’re calling in all of their favors and hiring mercenaries, assassins, and even private investigators in an attempt to discover the identity of their attackers. The PCs are one such group.
The pay is very good, since the Dragons are desperate.
The other crime syndicates are wary, since whoever is doing this appears to be well-coordinated and well-armed. The idea that one of them might be responsible has all of them on edge, and their guard is up.
Some of the crime scene is yet untouched. If the PCs hurry, they can get there and do their own investigation before other interested parties do. Careful searching will uncover a handful of spent bullet casings that match no known firearm. Digging further, if the PCs can find a source knowledgeable enough, will reveal that the casings are early tests of a new Army rifle intended for urban pacification.
Where it goes from there is up to you.