Aveline iirc....a neighbouring division was involved in some very high level corruption based upon a stately home. Last epidote of that series Endeavour and Thursday end up in that home in a shoot out. Thursday gets shot, but Endeavour is set up as the shooter by the corrupt high ups and ends up in prison. the truth never comes out, but Endeavour has been beavering away on it and suspects many boeis buried in the grounds, and they were found in the last episode, along with a nasty intervention by officers from that division beating Endeavour up a bit. (making Endeavour miss a vital "date" with Joa Thursday, about to get married)
Its now "all come out", and our heroes are determined at any cost to bring the corrupt officers to justice. At the last minute Endeavour opens the door to an officer who was presumed dead in earlier series.....(he had a romance going with the rather nice aptly named Constable Trewlove)