Megatron is only around for about 15 minutes. Years before the beginning of the movie he crash landed in the Arctic where he froze and was discovered by some explorers. The US government got hold of him and kept him frozen but towards the end, Frenzy sneaks in and stops the freezing device so he's able to break out. Then there's a big battle where all the Decepticons (except Starscream) and Jazz die.