Sadness (Phyllis Smith)’s Timeline and Summary

Sadness is one of the five emotions in Riley's head—although none of the other emotions really know why she's there or what she's good for.
Sadness touches one of Riley's happy memories, turning it blue—literally and figuratively.
She then prompts Riley to cry in front of her new classmates on her first day of school, creating a sad Core Memory.
When Joy tries to get rid of the sad Core Memory, a struggle ensues, and Sadness and Joy get transported to Long-Term Memory, along with Riley's Core Memories.
Sadness meets Bing Bong, Riley's imaginary friend from when she was a little kid.
Sadness, Joy, and Bing Bong try to take the Train of Thought back to Headquarters so Joy can restore the Core Memories and make Riley happy again. They fail.
Joy and Bing Bong fall into the Memory Dump, leaving Sadness behind in Long-Term Memory.
When Joy emerges from the Memory Dump, Sadness is MIA. When Joy finds her, Sadness makes a break for it, believing Riley would be better off without her.
Joy and Sadness return to Headquarters, and Joy tasks Sadness with removing the idea to run away from Riley's head. She's successful, and reunites Riley her with her parents.
Sadness assumes a larger role in Headquarters as Riley heads into the teenage years.