Under the Gun

Plot:

When a coded document hidden by a murdered bail bondsman gets Castle's imagination running wild, Beckett must steer the case back to the violent felons who populated their victim's world - only to discover that maybe Castle's crazy theories aren't so crazy after all. Complicating matters is the surprise arrival of Beckett's ex-partner (now a bounty hunter). As the two reunite over a history that Castle has no part of, Castle is forced to watch the relationship take a turn towards the romantic.