When the Decepticons start experimenting on Cliffjumper's body with the fabled Dark Energon, the Autobots pick up the life signal of their lost comrade. They hurry to the site, where they find a huge Energon mine, under Decepticon control.
When a weak life signal is received from Cliffjumper, the Autobots roll out and are met by newly-returned Megatron who has obtained a mysterious and powerful substance Dark Energon!