(Please don't read this text to the group verbatim! Nobody likes it when you read text to them out of a book. Read this beforehand and summarize in your own words.)
This adventure takes place in the town of Cedar Valley, which lies in a valley full of cedar trees. You guys are the "official" adventuring party attached to this town. You all know each other and trust each other; nobody's going to do anything crazy like murder their allies or defect to the Federation.
This year there is a severe drought in the valley. The plants and trees are drying out, and if this continues, there could be a forest fire. We've sent to the Dragon Empress for help, and in response, she is sending us an artifact from her hoard: the Orb of Storms.
(Created hundreds of years ago by a renegade dwarven druid, the Orb of Storms hates all civilization, especially dwarven civilization. The Orb expresses its rage by calling storms -- starting with torrential rain, and escalating to hailstones and tornadoes if unchecked. The Empire keeps the Orb in a state of mild irritation and uses the resulting rain for agriculture.)
We were all very happy, yesterday evening, when we saw a storm cloud enter the valley from the northern pass. We assume this storm cloud is the storm cloud that perpetually hovers over the Orb of Storms. But we woke up this morning and the storm cloud is on the other half of the valley -- it's hovering over the town of Freehaven, which is belongs to the human Federation of Free States.
So the mayor of Cedar Valley comes to you guys, basically at daybreak, and she wants you to go get our artifact back. It would be very, very bad if the Dragon Empress finds out it was stolen.
There's a river that separates Empire territory from Federation territory. It's a wide river, 120 feet across, at the bottom of a fifty-foot ravine. The riverbed is rocky and perilous. There's one bridge across the river, but it's guarded on both sides.
Fifty years ago, just after the Federation ambassador insulted the Empress, relations between our countries were really bad. We'd send adventurers across the bridge to burn down their town, and they'd send adventurers across the bridge to burn down our town, and it was -- it was just generally a bad situation. So now we have guards on the bridge, and we sort of have an informal truce not to send adventurers across to burn down anyone's town.
"But -- uh, we also sort of had an informal truce not to steal each other's artifacts. And so," says the mayor, "I want you guys to have these." She hands you each a vial of alchemist's fire. "I'm not saying you should use this, and I'm not saying not to use it. I just want you to have this, for your journey to Federation lands. Just in case."
(This alchemist's fire is not the "1d4 damage" thrown flask you would find in the 5e Players' Handbook. This alchemist's fire is serious business.)