When Castle is telling Ryan and Esposito about being abducted, Esposito dismisses "Valkyrie" as a military term. Although there's no military use in the United States, arguably the most famous military use of the word was in Germany during World War II; Operation Valkyrie was an emergency plan for a continuity of government that several high-ranking members of the Nazi Party used in the July 20 attempt to assassinate Adolf Hitler and negotiate a truce with the Allies.