Riley is a girl who has emotions like everyone else. Inside your head are five emotions, which counteract each other to be in balance. Luckily for Riley, Joy is the preponderant emotion in her life, which makes her always happy.
Now, Riley is going to suffer a change in her life. That change, which consists of moving, alters the balance of your emotions. That makes Joy no longer the emotion that sends within it.
At that moment, the rest of the emotions are altered and a question is asked. Why does there exist the Sadness? The truth is that it is a logical question, because it seems that the Sadness is useless.
In this way, the emotions unite to fight the Sadness. That fight is reflected inside Riley, who will suffer his emotions in an uncontrolled way due to the struggle within.