The Call of Cthulhu scenarios I've been running have all been one-shots because I've basically been mimicking the Mansions of Madness board game, or using short adventures I find online. It's been working well because my gaming groups are all currently locked into Pathfinder or Tales of the Valiant or Stardrifter campaigns, so all I've had time for are one-shots here and there. I was curious about what a Call of Cthulhu campaign looked like though, so when I saw a book advertising itself as an "action-packed and globe-trotting campaign" I immediately bought it.
The Two-headed Serpent is a 9-chapter campaign for Pulp Cthulhu, and I've read it cover to cover, but I've yet to run it. This is my review of the book based on its adventure design, structure, and plot.
This review contains minor and non-specific spoilers. The short and spoiler-free review is that this is an excellently written adventure, both in design and plot. The plot keeps you guessing for the whole campaign, and the design is robust enough that players have freedom to play as they please, and the game master (or the "keeper") always has enough information to ensure that the plot stays on track. Once I run it, I'll likely review it again from a game master's perspective.
There are 9 chapters in the book, and for the most part each chapter can be considered a distinct scenario that, when played in succession, builds toward the campaign's climax. There are notable exceptions, including Chapter 2: New York, and the 2 final chapters, which depend heavily on whether specific villains even exist by that time in the campaign.
The New York chapter is probably the most elucidating chapter for how the campaign is meant to be run. It's pretty clear to me that the authors intend New York to be the baseline setting for the campaign, the homebase location where the investigators spend their time when not abroad on company business. To begin with, the Caduceus Foundation in New York City is the employer of the investigators in this campaign, so there's always reason enough to return to New York after any single mission. The Caduceus Foundation is an organisation a lot like the real-life Red Cross or Doctors Without Borders. (Off topic, but relevant: Having benefited from the Red Cross in my real life very recently, I heartily recommend donating to it!)
Also, the book provides several adventure hooks to occupy the investigators with everyday adventures between official business. I don't think you'd necessarily have to add New York City adventures of your own in between chapters, but I think the authors expect you to provide a little padding to help pace major revelations about what's really going on in the game world. There's plenty of material in the New York chapter to help the game mater distract the investigators with mundane concerns, and it's done in a really clever way. I think I'd struggle to come up with incidental missions based around a charitable health organisation, but the book introduces a surprise faction that fits perfectly into the story, and could easily occupy the investigators while eldritch horror quietly tightens its grip in the background.
Another thing the campaign does masterfully well is getting out of the keeper's way. I don't tend to love an "open world" adventure, either as game master or as a player, but this book provides a "sandbox" that feels open while also being neatly contained. Each chapter briefs the keeper on why the chapter exists, specifically in relation to the campaign's storyline. Then it tells the keeper about each vital NPC in that chapter, and then it provides several suggestions about how the investigators might find themselves in the chapter's adventure. After the first chapter, the book never assmes that the investigators are still employed by Caduceus, so every chapter provides perspectives from each faction's point-of-view, plus the point-of-view of investigators that have chosen to ally with no faction. It's super helpful, not just to prepare for what stare the investigators in your game might be in, but also as a reminder of specific goals of each opposing force throughout the campaign.
I love this kind of adventure design. I think it's the correct method of designing a scenario for role-playing and wargames: Set up the problem, identify the variables, and then let the players play. This applies even to your bog standard dungeon crawl or hex crawl. The moment an adventure starts dictating important plot points that must occur in order for the plot to proceed, the scenario falls apart, and I've read a lot of adventures that make that mistake (and I've certainly invented my fair share of them, and have hopefully learnt my lesson for it.) The chapters in The Two-headed Serpent give the keeper the setup, the goals and intentions of NPCs or factions, and then gets out of the way. The adventure itself is for the players to create.
The campaign is written specifically for Pulp Cthulhu, a game mode for Call of Cthulhu designed for investigators that act more like action heroes than the typrical Lovecraftian protagonist losing grip on sanity. I prefer Call of Cthulhu, so I doubt I'll run this as a Pulp Cthulhu campaign (although it would depend on the gaming group.) Either way, the book allows for playing the campaign as a standard Call of Cthulhu campaign, at least insofar as it mentions the possibility. It provides a few suggestions on how to tone the adventures down enough for standard investigators to survive, but I think it would be relatively easy to improvise. You get a feel for investigator's tolderances pretty quickly in Call of Cthulhu, and I think there's plenty of mystery and intrigue in the campaign's plot to keep players engaged without a bunch of dnd-style encounters.
However, if it's dnd-style encounters you're looking for, then I concede that there are a bunch of great ones in The Two-headed Serpent. It's not just monsters, either. There are some action sequences that push player luck into the realm of impossible. There are ambushes, plane crashes, evil villains, terrifying monsters, and a bunch of other things I can't mention for fear of spoilers, and they require either the good fortune of low rolls or the sacrifice of actual Luck points to buy your way to safety. It's the kind of encounter design that feels unreasonable from the keeper's omniscient point-of-view, but that players somehow overcome (and maybe even survive.)
I love the encounters in this campaign, because they prove just how resourceful players can be when the stakes are unrelentingly high. It's the kind of sheer impossibility that forges lifelong memories. There's nothing like looking back at a scenario that, on paper, just isn't possible, and yet knowing that somehow your group of investigators got through it.
The Two-headed Serpent is an expertly designed campaign, with adventures that both preoccupy players and secretly progress the plot. It's partly up to the keeper to manage certain secrets that reasonably ought to come to a player's attention so that the "real" plot of the story can emerge, but refreshingly that's only a task of mitigation rather than narration. More than likely, players are going to uncover the secret plot points without prompting. The keeper doesn't have to reveal secrets in long expository NPC lectures. Sooner or later, the truth will out, and players have to deal with it whether it was something they'd long supected or something that surprises them.
That's the beauty of this campaign in summary, really. At every step, it enables in-game events to happen independently of the characters, and it's up to the players to take actions that influence the outcome. Sometimes they'll choose wisely, other times they'll choose poorly, but that's life, both real and roleplayed. I personally can't wait to run The Two-headed Serpent.