Here Calls the Spark of Primus.

"They were searching for new supplies of energon, and had gone to a planet that was off limits due to its geological instability. It was Starscream's idea. She barely returned with her life. Only Skyfire's shell returned. She was officially absolved of the blame, but two of her senior associates- her mentors- spoke to her harshly, and she was dismissed from her position. She joined Megatron's group a few days later." His emotions are shielded as he recounts the history, but he remembers well the angry, vengeful, and utterly broken femme that he'd seen that first day.