Whether a spoiler can land you in legal trouble hinges on the line between copyright and the First Amendment. A recent analysis in Reason walks through this tension, explaining that copyright law protects a specific expression of a story, not its abstract twists or turns. Restating a plot reveal in your own words is generally treated as unprotected idea, not infringement.
Fair use adds further room for spoilers when tied to criticism, commentary, or news reporting. The analysis notes, however, that copying an author's exact phrasing, or deploying spoilers for a commercial purpose, could cross into infringement territory. The more the spoiler reproduces the original text, the weaker the fair use argument.
The First Amendment supplies a separate shield. Courts have historically been wary of prior restraints, so banning a spoiler before it is uttered would face a heavy burden. The article suggests that no one “owns” a plot twist, but the specific words used to convey it remain the author's property. In practice, the legal risk depends on how you share the secret, not just what secret you share.