This adventure is an urban interlude, which should be run when the characters are in a large town about other business. The town needs to be big enough to have organised government and a sewer system... because the former wants the latter cleaned up!
It appears that recently some sewer workers were slain while cleaning up after a recent earthquake, and the 'city fathers' want to hire some adventurers to deal with the problem. If the characters don't take heed, it is suggested that some of the undead who are actually causing the problems pop up out of the sewers and attack passers-by or even the characters themselves until they do do something about it!
The adventure itself is a fairly straightforward 'dungeon bash' with a few twists - being a sewer it is very cramped - a 5-foot diameter circular tube, which will cause problems for both tall characters and anyone using a weapon that needs to be swung for full effect. The place is heaving with undead too, as the earthquake opened a way to a long-lost evil temple hidden in the bowels of the earth. The undead are enthusiastically coming out to infest the sewers and the city above. Worse, the temple is resistant to the usual magic that adventurers tend to employ - turning undead doesn't work as well as normal, and healing spells are also diminished in potency.
Although there's occasional mention of 'filth' and 'foulness' the fact that sewers are dirty, disease-ridden places is by and large ignored. You might wish to add a few choice diseases, and get characters to make Fortitude saves to avoid retching at the stench! The descriptions do not really bring over the feeling of claustrophobia that a small, dark tunnel ought to either - but perhaps adventurers are made of sterner stuff and don't mind such enclosed spaces...
Possibly worth dropping in as an interlude if your characters are wandering around a suitable town without much to do, especially if they enjoy dungeon-crawl fighting adventures.