Yes - intellectual property is a very touchy subject, and the burden of proof is heavy... (not only for stories, but for games also)
Basically, plotlines are virtually impossible to protect, as any possible variation has probably occurred in one or the other genre.
Two things are generally attributed to or associated with their creators:
- characters (from Mickey Mouse, Mario and other trademarked characters, to a Sam Spade)
- worlds/universes (the disc world, star wars, middle earth, etc)
Legally it is very hard to impossible (Elliot is right in that) to actually prove that you made an original creation, and vice versa. Especially the successful ones become 'common knowledge', and everyone is influenced - be it only to avoid similarities...