12 parts Complete Book 2 in the "Transformers: Legacy" series
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Now off Cybertron and left to their own devices, Bumblebee and Starscream led a decent life on Nebulous. It wasn't always the most comfortable - crammed into the small cockpit of a ship and hiding amongst the crowds - but they made it work.
Meanwhile, Optimus Prime worries for his friend. As the Crystal City government starts sending ships to look for the two runaways, he assembles his own team to search the stars for them before it's too late.
However, someone would get to Bumblebee and Starscream first, and they're not here to help any side.
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This story is also up on AO3, under the same title & the account Smokey_Prime9000. Warnings include robogore, kidnapping, and mental health topics, such as anxiety, panic attacks, & PTSD. Be aware while reading.