Note that this contains spoilers
The premise of this film is that each emotion inside a person’s head is represented by a character, and these characters work together, or not, to guide the person’s actions.
In the film we follow the characters inside the head of Riley, an 11 year-old girl who has just moved house to a new town, and has to face the challenges of re-establishing her life there. In this context, we primarily watch the relationship between Joy and Sadness inside her. Joy wants to take a positive approach to everything, and fights against Sadness, who naturally feels strongly in this situation. Much trouble ensues before Joy finally learns the lesson that it can be helpful to feel sad as a way to get through and deal with difficult situations.
I found the film quite surprising, as based on the trailers it had look like a comedy about the characters inside the head, whereas in fact the movie was quite tense and sad for a lot of the time, with the humour largely limited to what had been shown in the trailer. The idea behind the film is certainly interesting, and they clearly set it up for a sequel, but I would prefer a more light-hearted approach next time.