Will Farwicke is stabbed to death with his pool cue in his home with all of the doors and windows locked from the inside. Security footage shows his front door opening and closing but no actual person leaving the room.
When all of the security footage disappears and an unseen force tries to strangle Beckett at the crime scene, there are many crazy theories. Is it the devil, a demon or the invisible man.
Will's security key card turns out not to be for Greenblatt Insurance but for a top secret government lab where he was working on a cloaking suit. But when Will had trouble finishing his project he called upon an old girlfriend, Donna from his MIT days to help. Unfortunately once she did, he dumped her and stopped returning her phone calls.
Donna stole the cloaking suit and used it to sneak into Will's apartment and kill him. Castle and Beckett end up using a fire extinguisher to be able to find her and capture her while she is wearing the suit.
Javi finds out that Kevin is moonlighting as security at a male strip club in order to make ends meet at home. He gives his partner a pair of blue speedos with SECURITY stamped across them as his new uniform. Kevin thinks they should have a matching set.
Rick and Kate haven't made love since he's come back because each thought the other wasn't ready. Now that they both know how ready they are, they are continually interrupted. Once the close the case Kate show's Rick her own version of an invisibility suit.