Originally Posted by J.Howlett
In the first film, Sam and the Autobots assume that Megatron wants the Cube to transform human technology and take over the planet. That's their assumption.
Revenge of the Fallen technically retcons that but it makes it fit by saying that Megatron was one of the many Seekers sent to look for the Cube for the Fallen so that the Fallen could find the location of the Matrix of Leadership in order to reactivate the Sun Harvester to harvest our Sun's energy and help rebuild Cybertron.
I didn't get the impression that Megatron was a Seeker. The Seekers existed to find new stars for the Primes to replenish the AllSpark with, not to find the AllSpark; it was lost during the course of the Great War, some undefined time after most of the Primes had died. Also, the Decepticons weren't trying to find the Matrix with the AllSpark's knowledge, just the Harvester, because the Matrix would remain useless to them until it had deemed a new owner worthy of its power. As well, using the AllSpark's knowledge to find a Harvester was not part of their Plan A. Megatron was convinced that the AllSpark's destruction left them without the massive energies they required. Here's the dialogue exchange in question, courtesy of IMDb:
Megatron: My master, I failed you on Earth. The Allspark is destroyed and without it, our race will perish.
The Fallen: Oh, you much have much to learn, my disciple. The Cube was merely a vessel. It's power, it's knowledge, can never be destroyed. It can only transform.
Megatron: How is that possible?
The Fallen: It has been absorbed by the human child. The key to saving our race now lies within his mind.
It wasn't until the Fallen told him otherwise that he knew better.
For the record, Megatron's plan was clear to me. It's a retcon, but one that works. However, I see why people are confused; it's explained very quickly and subtlely by Megatron at the Lincoln Memorial, in a movie that's primarily obvious and repetitious about what it says. More time could've been spent unpacking it if there wasn't so much emphasis on other boring topics.