After Sherman bites a little girl named Penny Peterson for calling him a "dog", Ms. Grunion is determined to have the adoption agency reclaim Sherman from Mr. Peabody permanently.
Later on, after she did try to take Sherman away, Mr. Peabody's fatherly instincts got the best of him and decided to do what Sherman did to Penny the day before; by biting her arm. Ms. Grunion called the police to have him arrested while a hole was ripped in the space-time continuum.
In the end, she ended up being dragged back in time to the Trojan War period by King Agamemnon and the large duo were married at the Trojan Horse, while she became a kinder and happier woman.
Ms. Grunion is a dastardly, scandalous, underhanded, manipulative, and untrustworthy woman who is attempting to get Sherman taken away from Mr. Peabody. However, she does not actually care for Sherman's well being, she actually wants to ruin Sherman and Mr. Peabody's lives by making them miserable. She even went as far as reporting Peabody with a false accusation.
She was voiced by Allison Janney, who also voiced Gladys Sharp, from Over the Hedge, another film by DreamWorks Animation. She also voiced 2nd Dimension Charlene Doofenshmirtz, the main antagonist of the Phineas and Ferb episode "Tales From the Resistance: Back to the 2nd Dimension".