Spoiler Warning (but not as nearly bad as adaptation info)
Shooting for the Moon
The third movie builds upon surprising connections in human and Cybertronian history. It turns out that the race to the moon launched by President Kennedy was prompted to investigate Transformer activity. Optimus Prime's predecessor, Sentinel Prime, had developed a dangerously powerful weapon. Rather than risk it falling into Decepticon hands, Sentinel fled the battlefield only to crash his ship, the Ark, into Earth's moon. In Transformers: Dark of the Moon, Optimus and the Autobots are briefed on this history by a firsthand witness -- Apollo 11 astronaut Buzz Aldrin makes an appearance as himself in the movie.
The Decepticons and Autobots race to claim this lunar weapon, known as the Space Bridge, but a treacherous Sentinel Prime emerges alive, corrupted with visions of conquest. Optimus and Megatron must team up to defeat the ??????? before he can use the Bridge's powers of teleportation to transport Cybertron into Earth's orbit: the move would destroy Earth, releasing a massive wave of energy that Sentinel Prime plans to use to save Cybertron. The film's climax includes multiple battles with the largest cast of robot characters yet. Even Sam Witwicky acquits himself well in combat, as a shape-shifting ??????? built by the Autobots gives him some much-needed defensive firepower against the Decepticons.
The third Transformers feature -- shot and released in 3-D -- ends with an air of finality, and Michael Bay has stated it will be his last directorial effort in the franchise. The Transformers, however, will likely forge ahead in a medium that has proved so successful for them. What shape any future films may take, however, is anyone's guess.