This intro was shown before the first episode began properly, and basically gives everyone a quick catch-up to the events which happened before this series. It isn't on the Metrodome DVD set for the series, but it's on stuff like the Maverick releases.
Somewhere in the universe, there is a planet called Cybertron which is populated be robots which have the ability to transform from robots to vehicles and the like, hence the name Transformers. There are two sides at war with each other -Autobots, the good side, and Decepticons, the evil side. Optimus Prime leads the Autobots whilst Megatron leads the Decepticons.
Due to the amount of fighting going on, the Autobots realised that their energy reserves were getting very low and that they needed to find more. A group of Autobots led by Optimus Prime left Cybertron to find much energy, but they were followed by Megatron and a team of Decepticons in a ship of their own. The Decepticons invaded the Autobot's ship, but it ended up crashing into the Earth.
Both sides continued their war on Earth, with the Autobots trying to stop the Decepticons from stealing the Earth's energy for themselves. However, Optimus Prime was killed in battle.
Optimus was replaced by Rodimus Prime. Meanwhile Megatron, who had been badly injured in his final battle with Optimus Prime, was remade into and given and given a new body and a new name: Galvatron.
The war continued between the two sides, and the Autobots managed to reclaim Cybertron from the Decepticons. Optimus Prime was bought back to life, and a new Autobot base was set-up on planet Athenia, which is quite close to Cybertron. Meanwhile, Galvatron and the Decepticons were not going to give up so easily and re-grouped at their base on planet Charr, along with some new allies. And now, the Headmasters series can begin.