This is a one-off scenario, with pre-generated characters provided, set in a military hospital located just behind the lines in Belgium during the First World War. It's written as a purist adventure, but if you prefer to go a bit more pulp some ideas are provided to enable you to run it in that style.
There's some background that explains what is going on at the hospital, then it's on with the action, with the opening scene being in an ambulance convoy heading towards the hospital. Those characters who are injured and who will become patients at the hospital should determine with the Keeper what wounds they have and how they acquired them, this can be dealt with in a flashback scene (which may be held in reserve by the Keeper to be run at a dramatically-appropriate moment). For those who have been posted to the hospital, likely as medical staff, there's a slightly calmer introductory scene... but everyone ends up in the same ambulance convoy, although they do not know each other at this point. Then it comes under fire...
Assuming they survive the attack, everyone arrives at the hospital. It's pretty chaotic. Injured characters will have to be assessed and assigned to wards, those who have come to work here need to report in and be assigned their duties. There's also a rather excitable chaplain to deal with. From then on in it is a case of trying to figure out what is going on, with a host of NPCs to get to grips with, and various events and encounters as they figure out what is happening and how it can be halted.
Designed for a single evening's play it has the scope to be intense and highlight how even worse war can become if the Mythos gets mixed in. However, the mix of characters provided may not be ideal - it's hard to see how they will gel into a team - and an endnote suggests possible solutions mostly based on creating your own characters. Intended as a one-off, there are no thoughts for a follow-up - although it might possibly be used as a 'prequel' to a regular game: this is where the Investigators met and first encountered Things That Should Not Be, then skip ten years or so and they meet again to commence their adventuring careers.