We find out that the man from the Ring who called Casey last episode was James Keller (Robert Patrick). Through a flashback, we learn it was actually Keller that recruited Casey back when he was Alex Coburn. Keller sends Casey on a mission to steal something from the CIA.
At Castle, Beckman gives Casey, Sarah and Chuck the important mission to test the CIA's new 15 level security system. They get through thanks to Chuck and inside, Casey recovers the pill that Keller sent him in to get, but not without Chuck seeing him.
When reporting to Beckman, Chuck ends up admitting to seeing Casey steal the pill. Casey ends up getting arrested for treason and Sarah and Chuck head back to the 15 level security system to break him out. Just as they get to him, Keller breaks Casey out.
Now free, Casey tries to send Morgan in to get the pill back from the Buy More to give Keller, but Chuck and Sarah catch on and recover the pill. Casey admits it's because Keller is threatening Casey's former fiancee from his civilian days.
Casey sends Chuck to protect his fiancee, while he and Sarah go to confront Keller. Casey ends up killing Keller, while Chuck takes the pill (which removes fear) in order to flash and beat up the badies threatening the fiancee.
In the end, Beckman strips Casey of all his rank and makes him a civilian again. Chuck tries to convince Casey to go back to his former life and reclaim his fiancee. Casey tries to do the same and convince Chuck to get Walker before it's too late. At the end of the episode, Sarah seems to take Beckman up on her offer to relocate her to Washington D.C. as she arrives by plane.
Meanwhile, Ellie seems on board to do doctors without borders with Devon until she gets accepted to her dream of a fellowship at USC. Devon goes to try and get Chuck to get on his side and convince Ellie to leave with him. However, Chuck is gone and Devon ends up talking to Morgan and the two mutually discover they both know Chuck's secret. In the end, Devon tells Ellie congratulations on getting the fellowship and they'll make staying work.