It's difficult to adore an Add-on so much, which is why it means when I give this a ten it totally deserves it.
There is only one scattered letter in the game that has this expansion bearing any resemblance to the original game's story/characters, but aside from that it has its own original plot. You are a female protagonist who wakes up in a dungeon cell, watching a 'gatherer' monster (which is even scarier than the grunts and brutes before, probably because it looks more human) stagger away from your cell door. You find a phonograph next to you with a recording from a woman called 'Justine' who tells you that this is her 'cabinet of perturbation'. She has set up three 'test' chambers which contain a chained, impossible-to-reach victim and it is up to you to find out how you can save them then proceed to the next stage.
The gameplay and graphics are practically the same so I won't talk about it here. However, one of the most impressive and new things about this game is its degree of "non-linearity". You still have the same tight paths that you need to proceed through with minimal exploration in order to advance the story. However, it is entirely up to you on how to go about it. Only few of these puzzle items and scattered notes are at all necessary to proceed, but that's just the tip of the iceberg. Because these psychological puzzles are a test of character, it is up to you to decide whether to have the victims killed or not. There is always a way to "save" them but you can only either do that the way it has to be done or have them killed if you want to properly proceed to the next test chamber. If you managed to save each victim and collect all the notes and items you not only get maximum understanding of the story but you also unlock an alternative ending and hear Justine mutter three extra words at the end with strong resemblance to the ending Portal song.
Please gradually remind yourself for what I continue to say here, it's that the sound in this game is scarier than the original in every way, and I mean EVERY way. Those new patrolling monsters are as dangerous as the Brutes in the original game, and their music themes are scarier than the former. If you had no idea what level of horror to expect from this you would find it very hard to continue playing especially since this is a short game and any moments of death will have you starting from the very beginning. I'm also glad to say that the voice-acting is improved from before (which wasn't that bad either) and Justine constantly sends chills down my spine every time I arrive at the next phonograph during each of the stages.
It is great to see that there is more replay value found here in this expansion while at the same time having just-as-well-crafted-if-not-better storyline, gameplay and horror atmosphere. The lack of any saves may be an issue especially during the final test chamber which requires a lot of spot-on timing on solving the objectives while at the same time figuring out what to do next. Fortunately, I was patient even though I was slightly frustrated at having to start from scratch again at times. But this is a short game and one that was so engaging and interesting I just stayed with the entire ending credits when I finished. When you finished playing through the first game and experiencing it's three endings, you owe it to yourself to try this out this add-on. It is on the 'Justine' button which is found on the bottom right of the starting menu when you click to launch Amnesia: the Dark Descent.