Plastic Man first appeared in the first issue of Police Comics, back in 1941. Police Comics was published by Quality Comics, who went out of business in the 50s. Most of there properties & characters (including Plastic Man) were sold to DC, who reinvented their origins (sometimes numerous times per character), this post is about the original Police Comics origin. While the character is still under copyright by DC comics, the old comics published by Quality Comics have fallen into the public domain.
In this tale, Eel O’Brian is part of a gang that was robbing a chemical factory. Ell got shot in the shoulder, and some of the nearby chemicals spilled into the wound. His “friends” ran off and left him, but he survived his injuries thanks to a friendly monk, who also made him rethink his criminal ways. After discovering his new rubbery abilities, Eel O’Brian decides to put his “friends” behind bars where they belong. He pretends to still be plain old Eel O’Brian and accompanies the gang on a new heist, but then he slips into his (now famous) red & yellow costume and uses his new abilities to thwart the crime and send his former friends to jail. His former cohorts never realize that Eel O’Brian and the “freak” that caught them (whom they dub “Plastic Man” on the final page) are the same person. Eel continues to use his secret identity to gather info on crimes, which he then fights as Plastic Man.
If that bit about getting injured while robbing a chemical plant, absorbing chemicals into his body, getting mutated by said chemicals, being left behind by his comrades, and seeking revenge on said comrades sounds familiar, then you probably saw the 1989 Batman movie. I have no idea if it was intentional or not, but the writers of that movie clearly ripped off Plastic Man’s original origin story to use as the Joker’s new origin.
Now, without further ado, here is Plastic Man’s original origin story, in full detail: