We all know that Prime went out to seek "the Creators" and Galvatron is still on earth. There are still four autobots there that we know of and the dinobots. So, I'm thinking that since the creators wanted Optimus back, they probably already have some of the other "knights" (this is where Ultra Magnus comes in) and they're wanting to eradicate the rest of their creations, all transformers. So, they set out to earth, with Optimus and the other Knights in tow, (so they can see their fellow cybertronians destroyed) bringing along their destroyer, Unicron, (who can't really be a planet sized TF, so I'm curious how they'll portray him in the movie verse) to destroy earth and all its inhabitants. Galvatron, being Megatron reborn, knows that the only way to defeat Unicron is with the Matrix, so he has to strike a deal with the remaining autobots to free Optimus from the Quints, as he is too arrogant to be killed again and for good this time. That's all I've got so far, except for Galvatron turning on Optimus at some point..