Castle Bravo is an adventure set in the universe where Cthulhu runs around with abandon. Not a safe place for humans in the best of times. This adventure is set when the atomic bomb was exploded at the Bikini Atoll. Also not a safe place for humans. Combine the two and things get very, very bad. Very Quickly.
Besides the setting which is fairly unique and well researched the thing that makes this a great adventure is the character of the adversary. He is still human enough at the beginning so that one's sympathy is engaged. He did not ask to become a monster and at first he's more of a disquieting presence than a threat. He's not powerful in the traditional sense of a monstrous being but is powered by the weirdness of the quantum world. Kill him, and he'll pop up again later. He can be in two places at the same time. Oh and he has already seen parts of the future.
Not easy to kill that's for sure.
Oh and his touch might turn you into a monster.