Castle and Beckett become true believers while investigating the murder of an astrophysicist who seems to have died in space. Unfortunately, when they start digging a little too deep, it's not only their murder victim that gets an otherworldly visit.
Conspiracy, anyone? A mysterious government agent (played by actor-musician Lyle Lovett) that kidnaps, drugs, and interrogates Castle and Beckett for what? All for nothing since they had no clue what was going on anyway. I'm pretty sure that dealing with extraterrestrial life forms and UFOs are a little bit out of the NYPD's typical purview.
Of course there's more to it than the mysterious agent à la Mulder and Scully initially lets on, but its a lot less exciting than you'd think it would be.
In the end, Castle gets to threaten the agent with the only power true power he has, the power of social media! Who'd have thought that the threat of Twitter would really get a government agent to start spilling secrets?
Also, Castle's tip of the day: Always watch out for cyanide pills.