Annihilation (2018) explained. Spoilers abound.

Lena is an Army veteran and cellular biologist. She lectures students on cell division and evolution, highlighting that all life on earth is descended ultimately from a single, ancient cell. At first appearing to wax poetically about "the rhythm of the dividing pair, which becomes the structure of... everything that lives and dies", it's revealed the footage of cell division she has on display is actually that of cancer cells. Cancer isn't a foreign threat, it's our own cells dividing; which they normally do. What makes cancer pathological is that the growth is abnormal and unchecked. Essentially, their boundaries have been removed. It's not the material itself that's the problem, but the fact that it no longer conforms to a logical and constructive pattern of purposeful replication.

The tension between healthy growth vs. cancer, as well as the ambiguity of deciding which is which based on whether or not they seem to us to be beneficial or fit into an orderly framework, is carried over to Lena commenting on strange flowers which are remarkably composed of many different species genetically fused together: "You’d sure as Hell call it a pathology if you saw it in a human." In a human yes. In plants though? It's less clear, since one of the ways in which plants are very different from animals is that they lack our individuality and more obvious distinction between life and death. You can grow new plants from cuttings, you can more readily crossbreed them. These flowers aren't necessarily cancerous per se and could very well survive and thrive in their currently mutated state. They might even be desirable to us, if we could achieve the same effect. Florists would make a killing off of them.

The source of the Shimmer goes unnamed except for its accompanying soundtrack theme, titled "The Alien". It is truly alien; Lena has few answers when she's debriefed after escaping the Shimmer. We can't know what it wants, or even if it wants. Fan theories range from it being a Lovecraftian entity so different from us it's beyond our comprehension, to a biological weapon or failed medical technology created by aliens who lost containment of it; but I prefer to view it more abstractly as a pulsating blob of pure postmodernism. It's acting like a prism refracting everything, not just light, but radio waves and genes too; breaking things apart and removing the categorical boundaries between them; creating bizarre mixes and matches that should be impossible, freely combining all manner of plant and animal DNA. As Ventris comes to understand, eventually it will deconstruct everything down to its smallest constituents, essentially reducing them to nothing and utterly annihilating them. No more boundaries, no more logical patterns of purpose and order, and it has no limits.

The true limitless of the Shimmer's effect isn't spelled out for us in excessive exposition, rather it's plainly and clearly stated by Radek with a blunt finality that makes it ironically easy for some viewers to miss: "The Shimmer is a prism. It refracts everything." Yes, that means everything. Light, sound, waves, particles, DNA, and... everything. Including, it follows, the human mind. After all, our thoughts are not supernatural phenomena. They're manifested by our brains, which are comprised of biological matter and powered by electrochemical processes. Consciousness is still mysterious to us, and seems special and different compared to inanimate matter. But like everything we don't completely understand, it's merely a matter of us finding it difficult to fit it into a coherent conceptual framework. To the Shimmer, a mind is just more raw material to blend up and play around with, there's no distinction between it and anything else.

We see how the Shimmer appears to react to each individual's unique psychology. Aside from the characters who are taken out by an external threat, namely the ones killed by a nightmare-fueling mutant bear, the others appear to meet a fate of their own making or even choosing. Ventris, dying of cancer herself and realizing the Shimmer is causing them all to lose their minds, becomes fixated on a singular goal: to get to the source of the Shimmer and try to understand it while she's still herself. She has no objective beyond that, so when she succeeds in her task, she's finished. Literally. She explains empirically what the Shimmer is actually doing, and then promptly dissolves. Radek, who has a history of mental illness and self harm, has finally had enough struggle and suffering. She peacefully gives in to the Shimmer, plants begin to sprout from her self cutting scars, and Lena loses sight of her in the midst of a garden full of human-shaped plants. We assume she has transformed into one of them. The plants look like people because they've become entangled with human Hox genes, which are responsible for mapping out the basic body plan. Radek arrived at the same form from the opposite direction, being a human first who then became a plant. But does the road matter when the destination is the same?

Knowing it was likely a one-way trip, the secret organization studying the shimmer tasks Ventris with psychologically screening volunteers to enter the Shimmer. Logically, she found people with enough skill and stability to be reliable explorers, but burdened with some trauma that's removed their hope and survival instincts. Anya is a recovering addict with apparent anger issues. Josie has suicidal tendencies and self harms. Sheppard lost her child to leukemia, a tragedy that she admits essentially "killed" the person she was before. Ventris is dying of terminal cancer. Finally, we're eventually clued in to what would have made Lena's husband, Sergeant Kane, a candidate for a suicide mission: he had discovered Lena's infidelity with a colleague, and was rendered heartbroken. And Lena...

First, we have to ask why Lena cheated in the first place. Flashbacks of their marriage depict it as perfectly healthy and loving. Her colleague does remark at one point that Kane and Lena hadn't had a lot of time to be together lately, but that feels like a flimsy excuse considering how much they clearly love each other and are happy. We get the answer when Lena is talking to Ventris about self destruction, why people seem to have an innate tendency to make bad decisions or sabotage great opportunities and relationships for no logical reason. Ventris remarks that Lena, a biologist, is actually better suited to answer this question than a psychologist. After all, self destruction is baked into our very cells, which carry the seemingly disastrous flaw of senescence and eventual death. We've already learned early on that scientifically speaking, this doesn't need to be the case. If this error could be corrected, our cells could go on dividing normally without fail and we'd effectively be forever young.

So our answer to why Lena cheated is basically: that's what people do because people are deeply flawed, right down to the cellular level. It's reminiscent of a theological doctrine and not a satisfying, rational answer; but we're dealing with a situation that is so alien we're usually getting no answers at all, so at least it's something. The colleague Lena cheated with seems like a particularly annoying, Ivory Tower type who thinks he can explain away problems and dodge responsibility using sophisticated-sounding, but weightless rhetoric. He tries to employ pseudo-psychological manipulation on her when she breaks off the affair: "You know it’s not me you hate. It’s yourself." But Lena doesn't buy it, responding with: "No Dan. It's you too."

So Lena does hate herself for what she did, which to Ventris might look like it fits the criteria she's been using to screen every other volunteer. But unlike all the others, Lena doesn't want to die. She views death itself as a biological mistake, and she uniquely doesn't enter the Shimmer with the intention of it being a one way trip. She does so out of guilt, for the sake of her husband Kane who she already suspects was driven to it himself because of what she did to him. She wants to find a way to help him. As she explains during the debriefing: "I owed him". Lena is the one person to ever enter the Shimmer with her self preservation and sense of commitment to others intact. Thus, due to the fact that the Shimmer responds to everything within its influence including the human mind, she is able to leave.

Before she does, though, she encounters the Alien. Depicted as a 3-dimensional fractal, the Alien is wholly abstract and shows no recognizable signs of consciousness or biological life as we know it. It - mindlessly perhaps - generates a duplicate of Lena using a drop of her blood; and then Lena spends several minutes struggling with it, not realizing that rather than fighting her it's merely mimicking her movements. Lena's still fighting herself, literally now rather than figuratively. She finally catches on to this, hands the doppelganger a phosphorus grenade (along with finally letting go of, and passing on to her clone, her own self hatred and destructiveness), and manages to escape while the clone and alien structures around it burn down.

Back at the secret organization's compound, Lena is reunited with the Kane that had returned to her, knowing now it isn't the original version of her husband but another clone like the one she's just killed. She presses him outright on it, and he admits he doesn't think he's the real Kane. Then he asks if she's the "real Lena". She doesn't answer and quietly embraces him. Then both their eyes shimmer with the same ethereal, alien glow. Why doesn't she answer this question? The movie's not ambiguous, we clearly saw her clone die and her survive. But like the other rational "answers" in the movie we've gotten thus far, hers, while technically accurate, would be just as unsatisfying. Is she the same Lena? It's the same physical person who entered and left the Shimmer, but it's now been forever changed. The Alien has already infected her cells and begun changing her biology. Her eyes glow gold the same as Kane's clone. Like Josie and the trees, it ultimately won't matter which road each of them took, since they'll be arriving at the same destination either way.

And that's not just a material truth, it's a thematic insight about people and our lives. On a poetic level we're never the same person from one moment to the next, because we're constantly growing and being shaped by our experiences and relationships. Sheppard wasn't the same person after her child died. Kane lost himself when he found out his love had been unfaithful. Our very cells are constantly dying off and being replaced. We're always in a state of flux, and the only thing truly maintaining a sense of consistency in our personalities and physical existence is the conceptual frameworks surrounding them, the persistent patterns and narratives we're able to rationally fit all these things into and make sense of them. And that's precisely what the Alien, that big ball of seething postmodernism, was dismantling; which at first allowed boundary crossings that were bizarre and sometimes even beautiful, but without limits will eventually deconstruct everything into total oblivion. And now the Shimmer is gone, but it hardly matters since the changes have already been made and now the Alien resides in the affected flora and fauna, and copies made of it. It's in the ecosystem, which presumably will continue to head toward the same destination, albeit now taking a slightly different path. All roads lead to annihilation.