Castle was renewed for a 6th season on May 10, 2013. The season six premiere, Valkyrie, aired on September 23, 2013.
At first, Castle and Beckett work at keeping their relationship going while he is in New York City and she struggles with her new job in D.C., which turns out not to be everything Beckett had hoped for. When she is fired, Beckett returns to New York.
- Beckett accepts Castle's proposal of marriage and the job offer to work in Washington, D.C. for the U.S. Attorney General's office.
- Alexis returns home from Costa Rica with a new boyfriend, a fruitarian named Pi, who annoys Castle.
- Beckett must hurry to uncover who is behind a plot to steal a deadly toxin from a classified government contractor and poison the Secretary of Defense's wife before Castle dies as a result of being poisoned himself.
- Beckett is fired from her job with the U.S. Attorney General's office and regains her old position with the NYPD.
- Alexis and Pi move into an apartment together, much to Castle's chagrin.
- Alexis and her father come to terms over her living with Pi while working together to clear the name of a Death Row inmate. Alexis also makes peace with Beckett being engaged to her father.
- Castle is recruited to help a fan of his books solve the murder of her boyfriend.
- Beckett and Castle investigate a murder where the prime suspect claims to be from the future.
Weird Crime Scenes
- The former star of a 90s high school sitcom is found impaled from the hook of a construction crane.
- The prime suspect in the torture and murder of a parole officer claims to be from the future.
- An accountant is stabbed in the throat and hands and posed in an alley to resemble Jesus as he died on the cross.