The gang had decided that it was time to end the series. They did not want to limp out and be cancelled by the network - the show probably would have run for at least three more seasons.
This episode is absurdist perfection. The expected ending is Mary leaving WJM and Minneapolis to take a job at a network news department in NYC or Mary finds the perfect guy and get married and retire to start a family. But what they did is something totally unexpected and gut wrenchingly funny.
WJM gets bought out and the new owner decided to revamp the entire station. So he decides to get rid of the dead wood and keep the star performers. So that means get rid of Lou, Murray, Mary and Sue Anne and keep the great Ted Baxter.
This devistates Mary and decides to cheer her up by flying in her two best friends Rhoda and Phyliss. This scene will have you laughing and crying at the same time. The final scene with the cast leaving the newsroom for the final time is the best scene, you just don't know what to do!
As I said this episode set the standard for Series Finales and even Final Seasons.