My screenplay got optioned. Another writer was hired to revise it but instead wrote a treatment based on it. He copied/designed similar events. Now the production company plans to release me without money or credit. Is it legal?

Look, not an expert here, but, it is plagiarism. It is not rare for a writer to “lift” from another writer. But if they took recognizable parts you may have grounds to take legal action. Your work, by law, is already copyright protected automatically on completion of the work. But to take any action, first you need to go through the formal process of legally and officially copyrighting your screenplay - do that immediately. It is already protected, but to take legal action, the copyright has to be registered. Then whatever communication you have had, document it all, save everything, copy it all, print it all and organize it. You want all your ducks in a row and proof of your claim. And however you came to find out what they did, get a copy of that, and that treatment, or how to access it if you can. And if you want to go forward with this, you need a copyright attorney to file suit.

In the U.S. if it were just an idea, that isn’t protected. But a completed screenplay is protected, but it has to be registered with the U.S. Copyright Office.

.

Some interesting info and resources to check out: