The party arrive in a coastal town to discover that the local lordling has got himself killed trying to deal with an infestation of harpies. His distraught widow is glad to hear that there are adventurers in town, because he rather unwisely took the key to his strongbox with him, and she urgently needs to get into it. The task seems straightforward, but of course there's a bit more to it than that.
Pleny of notes are provided covering everything from scaling the adventure up or down, to wandering monsters and even why the grieving widow is so keen to have the key back. There's plenty of background material too, about the ruined cliff-top fortress where the harpies are based... and about what else is there that will present a greater challenge. There are also some adventure hooks to get the party to the right place if needed. The history is quite extensive, and it's left up to you how much you want the locals to know - and so relate to the party before they set off.
The adventure itself opens with the party in a row-boat heading towards the bottom of the cliff - the best if not only way to access the ruined tower. You'll need to come up with preliminary material covering their arrival in town and being hired for this task. There's a sea cave at the base of the cliff and steps leading up, but everything's fairly slick with spray. There are no wandering monsters here, but the place is not devoid of resident ones...
Up into the tower, and there is plenty to explore, monsters to fight and treasure to loot. As written, there's quite a lot of loot so if you are not inclined to be quite so generous there are notes on how to cut down without affecting the flavour of the haul. Some opportunities for interaction are provided for those who don't want to brawl with everyone they meet, but they will need to choose their moments wisely.
There's really a lot packed in here, a lot of exploration will be needed to figure out what is going on and how to deal with it. Several ideas for further adventures are presented, and there are notes on the surrounding area should the party decide to stay awhile. Altogether a well-construted adventure with plenty going on.