Spencer Reid is arrested in Season 3, Episode 18 "The Crossing," when he is apprehended at the airport after being framed for a crime he did not commit. However, the most significant and prolonged arrest storyline occurs in Season 2, Episode 24 "Lo-Fi," which continues into Season 3, Episode 1 "Tabula Rasa," when Reid is arrested as part of an undercover operation involving a drug-related case. The show explores Reid's vulnerability and the team's efforts to clear his name during this arc. Additionally, Reid faces legal troubles in later seasons, including when he is arrested in Season 10 as part of a major storyline involving Mexico and a tragic incident. The exact episode you're thinking of may depend on which arrest or legal confrontation you're remembering, as Reid's character faces multiple serious situations throughout the series that test him both physically and emotionally. These pivotal moments are among the show's most dramatic episodes and significantly impact Reid's character development, showing viewers a different side of the typically calm and collected genius profiler.