Very interesting start, a good setup, having the main character as a psychologist is quite a change for a scifi/alien movie, the creature looks great and original, the fx are pretty good level. There seems to be all the ingredients for a great movie.

However, once you're past the revelation of what's actually happening, it's completely fucked up by the fact that not a single action by any character makes any sense. It's completely ridiculous, and the only thing in your mind for the second half of the movie is "What ? Why the fuck are you doing this ? Are you insane ?"

For starters, yeah the choice of a psychologist is interesting, and makes total sense with the setup of the story. However, once you know what is going on for real, that's utterly stupid. What is she going to do ? [spoiler]Talk the creature into getting out of the body ? And how are either of them going to survive ?[/spoiler] In Arrival,they had linguists, that was original, and it made sense. Here less, but it would still make more sense than a psychologist. A biologist ? A chemist ? A zoologist ? A specialist on parasitism or symbiosis ? That would have helped.

Then there's the stupid addition of a useless _love_ story. She's a top qualified psychologist, and yet after 3 days and 2 discussions, she just fall madly in love with her patient (emphasis on the madly given what she does after that) ?

Then there's the other scientist guy, feeling threatened by her presence even if they are not in the same field and do not have the same job. He doesn't even want to talk to her so they can progress in their work, yet [spoiler]at some point he has a total change of heart and is also willing to die to help them escape ??[/spoiler]

Even if these actions already make no sense with nothing in the balance, they are incredibly **insane** when put in context. So the cosmonaut here is more or less in prison, ok. He is not badly treated, he's basically just not allowed to leave. On the other hand, he's [spoiler]inhabited by an unknown alien creature that goes out of his body without his control and eats humans[/spoiler], maybe that's a good reason to keep him locked in ?
It could be considered cruel that he doesn't know what's happening, but [spoiler]he even reveals that he has been lying since the beginning and is fully aware of it and having memories of the creature's action. Not only the fact that he lies is bad, by doing so he also conceals extremely important information for the study of the creature. It also reveals a very dangerous mindset that should trigger all sorts of red alarms for a psychologist[/spoiler].
So maybe wanting to help him is nice, but to do what ? [spoiler]They want to go to Moscow ? With the creature inside him ? Seriously ? And then what ? Leave the creature be ? Or just hold the guy in a less secured place in the middle of a huge city instead of a military base in the middle of nowhere ? It's even pointed out explicitly that we know nothing of the creature. That it could be just an infant form and the adult could be huge. We don't even know if it could reproduce. Or as it learns to stay out of the host longer and longer, if it would not just leave it at term and leave him to die.[/spoiler] The danger is incredibly HUGE and childishly simple to understand. And yet none of these high level scholars seem able to grasp it ?

And what about the actual [spoiler]escape ? They already know his life is in danger when the creature is out. Yet they try it. Knowing fully that a lot of soldiers will be killed. And when she finally realizes he can't live without the creature, instead of doing a U-turn and bringing him back to the creature, she just stops the car, and even gets him out of it, what ? trying to get to Moscow by foot ? What the actual fuck ???[/spoiler]

There are also included unrelated scenes with the kid, meant to [spoiler]make believe that's the guy's kid when it's actually her as a kid[/spoiler]. These serves absolutely no purpose in the story, or character development. There are just here for the "fool you, that was not what you though it was", but they are totally useless.

So great start, good concept, great realization, good fx, great actors, but shitty movie that really turns into irritatingly stupid nonsense.