This will be about the entire "The Knot" series, as this game contains its conclusion. Overall, I think the games are rather interesting both as a vaguely meta-fictional exercise and as stories in of themselves, and worth playing. It might be best to play them by opening all three games in the browser simultaneously. In general, the presentation is nice, but I am extremely not a fan of the slow auto-advancing text, which is the entirety of the ending sequence.
Spoilers for the ending and for the story overall: (Spoiler - click to show)"The Knot" is a tale about power, storytelling, and alternate worlds. Each story within The Knot contains the same character names and elements in different contexts. They are all about conflicts between two central figures: Chirlu and Ilfane, who are entangled with an artifact called the Knot, supposedly a source of ultimate power. Sometimes Chirlu is presented as a "good" character, other times as an antagonist. In "Terror" he is an evil sorcerer, in "Adventures" he is a Nazi archaeologist, and in "Incident" he is a benevolent scientist. Ilfane is more of a mystical concept than a character; it is a location in "Terror", an ancient autarch in "Adventures", and an evil alien race in "Incident".
(warning: extremely basic and naive analysis ahead - this is like, my opinion only)
Overall, Chirlu and Ilfane represent the conflicting natures of rationality and mysticality/tradition. Neither are totally "good" or "evil; rationality can be put in service of evil as easily as it can be in service of good. However, both figures always seek out the Knot, which is supposed to be the source of their ability to do the ultimate good for the galaxy, or to give themselves ultimate power. Chirlu especially always seeks out the Knot to achieve their ends, conditioned by the societal conditions in which they are raised.
The Knot itself is treated as a representation of power in some way. But the conclusion of the story shows that the Knot does not even exist; it is totally incapable of the feats ascribed to it throughout the course of the stories. This can be interpreted in multiple ways. The Knot is a video game, and the solution to a simple video game puzzle will not give one the power to change the world or to fight Nazis. Similarly, it could be a commentary on the impotence of media in general to bring change. Or on a simplistic, one-off solution to achieve societal goals, sought by progressive revolutionaries and fascists alike. They enter the halls of power, only to find the halls empty.
As a game, the Knot is not particularly challenging: the solutions are given explicitly, and labeled as such. Finding The Knot is not a challenge. But the Knot is ultimately hollow. It is certainly not the ultimate source of power. It might not even exist.
Solarium is one of my favorite pieces of interactive fiction, or any fiction really. It's another of those stories that I find myself returning to over and over. Every time I read it, I feel like I discover something new, another layer to the story or a reference I didn't understand. If anyone hasn't yet, you might want to play it for yourself without reading this review; my description won't do it justice.
Insofar that Solarium is primarily about any one thing, it's about the horrors of the Cold War. In their quest for supremacy over the Soviet Union, Americans turned to esotericism, with a magical archaeological discovery that promises to protect them from nuclear retaliation, thus breaking the game theory of Mutually Assured Destruction and allowing a first strike. Of course it doesn't work that way; actually they awakened an ancient evil that wanted to destroy the world. And it did. But that's just the surface; there's a lot more to the story.
Solarium is a hypertext story told nonlinearly and nonchronologically. It is a mystery story where the mystery is from the perspective of the reader, to find out what happened and why. There is a root node taking place "after mutually assured destruction" and many flashback segments (can I call them storylets?), each associated with a substance, unlocked by going through other flashbacks (which are treated as alchemical rituals). Through these flashbacks, the player discovers the history of the protagonist and their relation with the events that lead up to the nuclear apocalypse.
There are two endings decided by a final choice at the end. It makes sense; everything that comes before is flashback to prepare the player for this final decision. Spoiler description of the plot and ending: (Spoiler - click to show)The plot takes the cold war and moves it to cosmic dimensions. The protagonist is the reincarnation of an ancient godlike figure, and both his lover(?) Annalise and the Archon (the spirit contained inside the magical amulet) are also reincarnations, playing out an ancient cosmic drama between good and evil. Their bodies are no matter; the Archon takes over the president's body, and the protagonist is reincarnated as men and women, including a priest and a soldier. All of them are endlessly lonely through reincarnation, and the Archon tries to attract the attention of the Creator by acting up, by causing so much mayhem and evil that God is forced to notice him. Meanwhile Annalise is as pure good as possible; it's implied that she is the reincarnation of Jesus. The ending is with Annalise dying permanently, and the protagonist can either join her in death or keep on living. In the latter ending, eventually the protagonist finds the Archon's amulet again, because they're so lonely and need a companion.
The game is littered with a complex array of references, from literature to religion to real-life Cold War history complete with actual documents. Maybe it's only impressive to a relatively uncultured person like myself, but I thought it was incredible, and made me look up a lot of things on Wikipedia, like the real life Project Solarium, the use of LSD by the CIA, Gnostic religions, and the history of alchemy.
More generally, the writing is incredible (in my opinion). Every sentence just feels perfect. I don't know how to talk about it without gushing. The nonlinearity and gating are usually well thought out, and work to pace the story and control how and when the player accesses certain content. Most of it is pretty easy to navigate, but there were a few moments where I wasn't sure how to proceed. But it turns out that some storylets can or have to be repeated multiple times after getting new substances.