Despite the critical acclaim this film has received, I personally found it to be quite perplexing. While I'm a big fan of Alex Garland's previous works, such as Ex Machina and 28 Days Later, I found this particular film to be lacking in coherence when it comes to its plot and the way it presents its final revelation. The inclusion of an unnecessary cliffhanger also left a bad taste in my mouth....
Really bad, I mean just awful.
Spoil incase you couldn't watch all the way through...
Essentially, the movie explores basic biological concepts related to living organisms, including cells, mutation, DNA, and so on. It introduces an unknown alien life form that is attempting to adapt to our planet's environment by merging with living organisms in the bacterial and animal kingdoms. As viewers, we are given a first-hand look at the genetic mutations that result from direct exposure to this entity and its environment. We see how it expands and takes over our natural environment while creating new, mutated life forms in the process.