K.C. and her family of spies, along with her BFF Marisa, are on the run from archenemy Zane in Rio de Janeiro when they must take on another spy like non before - the fierce and slightly bizarre enemy agent Sheena.
Unbeknownst to K.C., Marisa leaks classified information; Judy's disagreement with Diane over the tooth fairy jeopardizes the sophisticated robot's disguise as an everyday 10-year-old.
When K.C. tells her family and The Organization that Brady is an enemy spy with The Alternate, the Coopers secretly work with Marisa to set a new plan in motion to catch him.
When Judy is due for an upgrade, the Coopers reminisce about their great times before they have to delete her memories and return her to the Organization.
K.C. realizes she misses Darien and goes to tell him, but he reveals he has a new girlfriend. K.C. lies and says she has a boyfriend and the two agree to go on a double date.
The Coopers prepare for Poppa Earl to visit with his new fiancé, Betty. He surprises everyone with how happy he is, but K.C. recognizes Betty from an unsolved case from 1977.
K.C. is assigned to protect a foreign diplomat at a university ceremony and must go undercover by joining the school's drumline to secure a key position next to the diplomat.
Byron waits for K.C. to respond and she says she's not K.C.. but her cousin, Cassandra. Byron suggests they all go to dinner, so K.C. breaks Bernice out of jail to play her cousin.
K.C. decides to save Ernie since it was her fault the barbecue failed. They learn the Bishops have planted listening devices and someone called the "The Mask" wants the weapons sold.
