The woman that would become Plasma has an unrevealed past before became a herald of Galactus. 
 
She along with several other Heralds were worshiped by the Inhumans of the Heroes Reborn world. Plasma along with the other heralds were sent to Earth in order to prepare for Galactus' coming. After facing off against the Fantastic Four, she was sent to Monster Island in order to place a energy capacitor in order for Galactus to convert the Earth's energy. She was defeated by the Avengers. Galactus was still successful and Earth was destroyed. Doctor Doom traveled back in time to try to prevent this from happening again. 
 
Plasma was still to set a capacitor in Los Angeles and was faced by SHIELD. After severely damaging the SHIELD Helicarrier, Nick Fury sent it into a suicide run into the capacitor, sacrificing himself and killing Plasma in the process. But again, Galactus consumed Earth and Doom traveled back in time once again. 
 
Plasma and the other heralds arrive on Earth and face the Fantastic Four. The heralds kill each of the Four, Plasma finishing off Mr. Fantastic. Once again, the remaining heroes of Earth cannot stop Galactus, and Doom travels once more to 24 hours in the past.
 
Plasma is not seen in the next time period and Galactus is finally defeated by the Silver Surfer.