Fear what's inside.
Why it is hard sci-fi
A team of scientists enter the Shimmer, a steadily expanding zone in the southeast US where biological boundaries between species are dissolving. The film is procedurally rigorous about the team. A biologist, a paramedic, a physicist, a geomorphologist, and a psychologist. Each has a role in their own area. The Shimmer is presented as an ongoing experiment rather than a curse, with measurable effects on light, RNA, and behaviour.
Science inside it
Cross-species genetic transfer, refraction-mediated reorganisation of biological tissue, and a working model of the Shimmer as a refractor of all signal types, including DNA. The films plant-shaped human and bear-with-human-voice beat are coherent within that rule, since the Shimmer is mixing all signals it receives.
Spoiler alert
Lena reaches the lighthouse, encounters a shape that mirrors and improves on her, and triggers a phosphorus burn that collapses the Shimmer. The film closes with the implication that what walks out of the lighthouse may be the new Lena, and the husband who returned at the start of the film may not be the original either. The film commits to that ambiguity rather than resolving it.