The pollution in Fair Haven is getting really bad... there's this blue mist that is killing some of the inhabitants and sending many of the rest insane. It seems to be coming from the sewers, so of course any adventurer worth the name will be straight down there to find out what's going on.
The DM notes begin with an overview of the adventure, there's an encounter list, scaling information, and some notes on how best to get the party involved. This adventure can be run as a sequel to DCC7: The Secret of Smuggler's Cove, but if you don't want to do that there are a couple of other ideas to propel them in the right direction. There's also plenty background material to make sure that you're clear about what's actually going on and how it came about... underpinning it all is a devilish plot to turn the world into another plane of Hell!
The adventure proper begins with an investigation into the noisome sewers of Fair Haven. They're cramped, smelly, and there are always traces if not pockets of the deadly blue mist. Also, there are wandering monsters to contend with. There is also a magnificent puzzle/trap that appears to be the key to dealing with the mist. This includes a riddle that appears almost out of thin air, for which a handout is supplied. It's noted that a particularly harsh DM might show it only to the player of the character who sees it, and snatch it away after the 30 seconds for which it appears (I had a DM play a similar trick on me once... the poor dear didn't know I have a near-photographic memory and just wrote out the message that had faded before my character's eyes!). There's a lot more to find, to fight, and to puzzle out down here. And the smell never gets any better!
As the riddles of the sewers are solved and the inhabitants put to the sword, eventually the party should open a portal to... well, somewhere else. They get sucked in, it's unavoidable. It's a dimensional prison, caging something that really, really ought not to be allowed out; and it's full of cryptic traps and puzzles. They are partly to keep the inmates in and partly to stop anyone breaking in to rescue them, and there's a third, darker, purpose (which could lead to further adventures...). A mix of aggression and cunning is needed here. Nothing is what it seems, but all is extremely dangerous. Indeed, it's likely that not all the party will survive. There is layer upon layer, you think you've reached the end and yet another level opens before you...
The end is suitably dramatic, with the party returned to a sunny Fair Haven with an enigmatic voice ringing in their ears. There are some ideas for follow-up adventures too. If you want a wild ride of deadly danger with the well-being of the very world at stake, look no further!