I actually started writing a one-shot for this one...and will finish it one day, but I decided instead to run the excellent short adventure for Delta Green, The Last Equation.
For those unfamiliar with Delta Green, it was a setting originally published about a year prior to the start of the X-Files. It basically takes UFO and Paranormal mythology in a different direction than X-Files, adds a revamped and updated for the present look at the Cthulhu Mythos, throws it in the blender with contemporary UFO and Paranormal mythologies, and hits frappe.
There are several elements that make a good Call of Cthulhu game. First, the menace needs to be one that can't easily be comprehended by the players. Second, it needs to have an ancient, murky origin. Third, there has to be a real, palpable sense that players can die from this threat, even if they do everything perfectly. The Last Equation fits these molds perfectly and like most great horror tales, the monster isn't half as dangerous to the characters as the characters themselves are.
My work on the game consisted mostly in generating pregenerated characters. I'm using the excellent HeroLab (with the Call of Cthulhu data files suitably modified for Delta Green), and generated 10 characters...out of those, the players will choose six. Due to a precondition of the game, there's a very likely chance some of these character may or may not be played based on the game instructions. Depending on the characters the players select, this could be a very different game.