This is the ultimate Call of Cthulhu Advneture for your group, with HPLHS's legendary propo set to go with it. Everything you need to starte the adventure.

Masks of Nyarlathotep is a series of sequential adventures set in the 1920s that pit the investigators against Nyarlathotep, the Crawling Chaos and Outer God

You get the following:

Call of Cthulhu: Masks Of Nyarlathotep Slipcase Set (Books and GM screen)
HPLHS's prop set (The H.P. Lovecraft Historical Society)

Description from their website:

Since its original publication in 1984, the "Masks of Nyarlathotep" supplement for Chaosium's Call of Cthulhu® has been hailed as a classic among role playing gamers. Now, as Chaosium prepare to release a full updated version of it for the 7th edition Call of Cthulhu rules, the HPLHS is releasing a very special collection of props to enhance your role playing fun.

We've taken all of the handouts, maps and props from the game (and a whole bunch of bonus stuff Chaosium doesn't include) and created them as highly realistic detailed props. Why would you want our props instead of the ones that come with the Chaosium's new edition of the game? We're glad you asked.

We believe great props make for great gaming. So we've taken our 30 years of experience in making props for games to create this set. Because we are able to break free from the confines of the printed supplement book, our props can be printed in larger sizes, on different kinds of paper, and with additional details. For example, our newspaper clippings are really printed on one of the few remaining newspaper presses in the country. They have back sides with genuine copy and advertisements from vintage papers. We've created custom fonts based on real old newspapers to capture the feel of vintage typography. They look right, they feel right and they even smell right! Plus, our props don't have to be printed in a book or as a PDF - as a result we can make props that are way bigger than you'll ever find in a game supplement.

So, what do you get?

All 109 prop documents, handouts, and maps from the Chaosium game supplement, designed and printed in the most realistic manner possible
Twenty-one more bonus props which we've included because we think they'll make your gaming even more fun
An Ediphone wax cylinder case that holds a custom USB drive containing eight props that are audio files recorded by professional actors - play them for your players! (We include transcripts to aid their memories.)
A set of 6 Nansen Passports issued by the League of Nations for your players and a set of custom passports stamps which can document your players' travels
A Chinese scroll containing mystical secrets in Mandarin