Justin, an art transporter, is shot and killed on a parking deck. Mia Laslow makes an anonymous phone call to the police when she sees the masked gunman leave carrying a briefcase. Ryan and Esposito track down her address and bring her into the 12th but she’s not happy.
Mia has Hyperosmia which is an extremely heightened sense of smell. She didn’t see the killer’s face but she know what he smells like. But Mia doesn’t want to get involved. She normally stays in her apartment because the outside world of smells overwhelms her. Castle is the only one who can convince her to help with the case.
Hayley turns up at the 12th with information concerning the case. Justin’s briefcase held a Van Gogh which was lost during WWII, later found and recently sold at an auction in Zurich for $65 million dollars. Someone must have stolen it when they killed Justin but Justin wasn’t suppose to be transporting the painting, someone else was.
Esposito and Ryan both get their Sergeants exam results. Esposito passed but Ryan didn’t and he’s devastated. Later, while chasing down a suspect, Ryan accidentally shoots Esposito in the butt. It’s only a flesh wound but Esposito is upset. The two continue to bicker and Beckett threatens to send them both to therapy if they can’t figure things out.
Mia tells Castle that Beckett’s pheromone output goes up whenever she’s around him. After spending time around the two of them she tells Castle it’s obvious how much Beckett loves him. Mia also turns out to be a huge fan of Martha’s. Castle gifts her two tickets to Martha’s new play.
Mia and Castle help track down Liam, the man with the briefcase but the painting inside is a forgery. Later, another man, Lorenzo, comes to Castle’s loft with a gun. He knows Castle is Beckett’s husband and wants him to help get the painting from evidence. Castle fights him off, but not before accidentally breaking Mia’s nose. Turns out Lorenzo had no idea that the painting in evidence was a forgery. Later Castle gifts Mia two tickets to Martha’s new play as an apology.
It turns out that Justin’s brother, Graham was not only an artist but a gambler. He made the forgery but then killed his brother when he found out he wasn’t going to sell the original for profit.
Later, Beckett returns to the loft to collect more of her things. She smells one of Castle’s shirts and takes it with her, leaving one of her NYPD t-shirts behind in its place.