The plot of the movie is quite familiar to those who have seen the previous two parts – whatever happens the Autobots win! Decepticons come up with the real cool robots (as one character admits in the movie), but end up on the losing side as the good ones rip their spines apart – literally! And the weakness of this franchise is that this plot never really thickens.
There is a great new character (Sentinel Prime) introduced in this movie. The gist is, this great one was having a cargo load of pillars (thingies that can open a huge tele-port) and crashed into our moon and went to hibernate mode (for nearly half a century). The movie even goes on to say the main agenda for first manned mission to moon was to check out on this crash – typical of Hollywood!
The Decepticons know of this crash while Autobots don’t. They even managed to scoop up most of the pillars and just need Sentinel (they do come up with real cool names) restored. You see, he’s the only Cybertronian who knows how to open the tele-port gates using these pillars. The only way they can bring him back is via Optimus himself, for he has the Matrix that can be used for such a restoration.
How can one expect a Prime to assist the Decepticons once restored? Answer lies in the fact that Sentinel was already in a deal with Decepticons when Cybertron was about to be destroyed – that they will restore their planet together and rule again (like gods). Like all big plans – things get waylaid, there are ego clashes blah blah.