Better Late Than NeverLink Discuss (via Robot Wisdom)
Some bad guys have arrived and done some bad guy things. The PCs were none the wiser. The bad guys have now made good their escape, and the PCs have caught wind of it in time to chase them down before they make it back to their lair, their home nation, behind enemy lines, etc.
Common Twists & Themes: The bad guys escaped by stealing a conveyance that the PCs know better than they do. The bad guys duck down a metaphorical (or literal) side-road, trying to hide or blend into an environment (often one hostile to the PCs). If the bad guys cross the adventure's "finish line" (cross the county line, make the warp jump, etc.) there's no way to pursue them beyond it.