what exactly happens in the final episode of 3rd rock from the sun? what happens to Mary and do they get back?

Answer 1

After Mary sees Dick change Liam (a malevolent alien played by John Cleese) into a monkey, Dick reveals his true identity to Mary. The Big Giant Head finds out and tells them that their mission is over. Sally helps Don realize what it means to be a police officer and Dick leaves Mary forever.

The ending scene features the aliens singing goodbye in their Rambler and disappearing back to their home planet. Later, in syndication, an alternative ending was shown with Dick returning in the nude and abducting Mary.