Captain Lennox: So why Earth?
Sam Witwicky: It's the All Spark.
Keller: All Spark? What is that?
Sam Witwicky: Well, they came here looking for some sort of cube-looking thing. Anyway, Mr. NBE-1 here, aka MEGATRON...
[glares at Simmons]
Sam Witwicky: That's what they call HIM... who's pretty much the harbinger of death, wants to use the cube to transform human technology to take over the universe. That's their plan.