BBC release trailer for Line of Duty season six. (BBC) The trailer for the highly anticipated sixth season of Line of Duty has dropped ahead of its release later this month. The hit BBC show will make a return to BBC One on March 21 with a seven-part season, making it the longest series to date. News of the its return has left fan wondering what could be in story for AD-12 in the show’s latest instalment. Until now, details of the upcoming season have been kept under wraps. The new trailer was posted on social media by the official Line of Duty Twitter account with a caption that said: Fellas, try to contain yourselves…”