Castle stands by his wedding proposal even as Beckett announces she already enrolled in a FBI Washington task force training program. But he can't resist overstepping his promise to keep out of her work when a top-secret case comes up. Snooping on the presumed trail of clever high-tech robbers, Castle ends up contaminated with a toxin that has already killed a suspect while he was in the same car. Meanwhile, Alexis returns from Costa Rica early with Pi, her charming 'fruitarian' boyfriend, who now resides in the Castle home, being stuck after losing his passport.