Hellbrood: Countdown to Invasion is an short introductory campaign leading into the main Hellbrood campaign detailed in a future book. The first part of the book has a brief history of the campaign world and how the heroes fit in it. Followed by some new setting rules, races, edges, hindrances, 6 adventures and then finally the NPCs necessary to complete the story arc.
The writer's intention is to make the PCs part of a comic book crossover event like "Blackest Night" or "Crisis of Infinite Earths" and they accomplish a lot toward that goal. The adventures are straight forward, but tied to the setting and future events. I assume a lot of the pay off will happen in the next book. Playing this without going into the main Hellbrood campaign I don't think would work, unless you like your superhero tales to end on a downer.
My only real criticism is the length of the book, its too short. I would have liked to have seen more of the world built before it is torn down. Makes for more of a sense of loss.