One pet peeve I want to address up front: in Korean movies with English dialogue peppered in (like this one) no one bothers to subtitle the English, just the Korean. I want to read it all people.
The Host is about a mutated river beast that goes on a rampage one day, stealing a little girl from her family. The father (Song Kang-ho), with the help of his own father, brother and sister goes after the girl to rescue her despite the entire country thinking he is carrying a deadly virus. We follow both the girl as she attempts to escape the monster's den and her family as they try to evade the government and find locate the girl. There is action, comedy, some intrigue and lots of sadness as things don't always go so well.
Like most Korean movies, this one kept me guessing as to where it was headed the whole time. I completely guessed wrong about the outcome of this movie. Song Kang-ho is, apparently, my favorite Korean actor as he has appeared in Thirst, Age of Shadows, I Saw the Devil and pretty much any movie I have liked from Korea. His character is almost mentally challenged in that he is like a child most of the time but prone to sleeping fits, too. The educated brother and the archery-champion sister make a pretty badass team, also.
The monster effects are strong and the storyline about the hunt for the monster's cave is compelling. Unfortunately, there is this weird secondary plot where the U.S. government is creating a panic based on a virus that doesn't exist just so it can unleash a chemical agent in a populated area of South Korea. It seems like a stab at the U.S. entering Iraq based on phony intelligence but the point never really gels for me. The more the movie focused on the family, the better it was.
So, I would recommend the Host with a few reservations.