I won't go into a deep summary of the plot, except to say: Flawed, over-proud, god-like super hero is banished to modern day earth and left to figure out his own way home, meets pretty scientist with whom he strikes up a very mild romance, engages in lots of action and violence both in the mythological world and on earth.
This was a truly lighthearted and fun popcorn flick. The actors portraying Thor, Loki, and Heimdall gave outstanding characterizations. Brannagh was an interesting pick as director, but he delivered, even with his characteristic multiculturalism (Heimdall usually known as the whitest of gods was extremely well played by Idris Elba, a British black actor and Hodun was played by rising star Japanese actor, Tadanobu Asano). Humorous and action packed, I would say the biggest flaw in Thor was the casting of Natalie Portman as Thor's earthly love interest. Her role could have been played better by a weaker actress.
What I love best about Thor is I can send my teenagers to see this flick with absolutely no qualms. Very rarely does Hollywood put out a movie truly appropriate for teens. Due to plenty of violence, and one incidence of mild profanity, I would recommend this movie for teens, and adults. The USCCB rates this film A-II, adolescents and adults.