Re: Confused about Megatron's plan
Via the second film, all the Seekers that have been on Earth for a number of years were looking for the AllSpark.
Megatron was also looking for the AllSpark as well, via the first film. And since the Seekers were all working for the Fallen to find the AllSpark, it only makes sense for Megatron to be one as well, considering he answered to the Fallen in the second film.
And while the AllSpark was the knowledge of their race and a way to rebuild their species and planet, the Fallen needed the knowledge of the AllSpark to find out the location of the Matrix of Leadership, which is the KEY to activating the Sun Harvester.
So, in theory, that's Megatron's mission. Find the AllSpark to give to my master. The Fallen had the grande plan of using it to destroy Earth's Sun and harvest it's energy to rebuild Cybertron.