Firmly placed in the middle of a linked series of adventures, the Introduction sets the scene to a sufficient extent that if you have not played the Against the Giants set of scenarios first, you could summarise to explain to your players what is known so far and why they might want to undertake this adventure. Basically, after the climactic battle at the end of G3: Hall of the Fire Giant King, it became apparent that the Drow were instigating the trouble but many of them fled down a tunnel... so, are your brave adventurers going to follow? Assuming they do (or are otherwise interested in pursuing this adventure), there are a few notes on the preparations which they ought to take, and the reminder that the adventure, while rated at 10th-level, assumes a large party of 7-9 characters and that smaller parties should enlist the help of local elves.
The first part of the adventure involves actually getting along the tunnel into the complex below, and while there's a lot of detail about 'wandering monsters' that may be encountered, the DM will either need to thoroughly prepare what will happen when and where, or be happy running varied encounters on the fly as seems appropriate. There are a couple of fairly set encounters before the end one of the first part (the 'Descent'), but it's mostly a case of wandering along subterranean passages and probably brawling with whomsoever you find.
The second part (originally Shrine of the Kuo-Toa) draws upon what the characters should have learned from the end encounter of the Descent part, although it's not clear how the characters work out much more than that a certain race is involved, rather than figure out what they are doing. Again there are a couple of set encounters and a lot of scope for random brawls with wandering monsters with no plot relevance, as well as a final set-piece in the Shrine itself.
This is noteable as it is the first introduction to a race that has become a favourite in Dungeons & Dragons, the Drow or Dark Elves. In terms of plot, the adventure is quite weak and linear with little scope for interaction save at sword-point; but for those looking for a dungeon crawl there is ample scope although as mentioned earlier the DM needs to prepare comprehensively (or be happy running truly random encounters) as a lot is left to his imagination. A classic early adventure.
[4 of 5 Stars!]