It's very difficult to copyright actual code, which is why virtually any program you buy has a clause in the license that forbids you from decompiling, disassembling or reverse engineering in any way. That way, if they find strings of their code in your program, they can do you for breaking the license agreement.
Artwork is completely protected by copyright. This includes photographs and the use of an image as an underlying image. The same is true for music.
Text is protected under the same laws as other text. You may quote, providing you name the source, and your quotes may not be more than a certain length. You may also copyy up to 30% of a single article (not exceeding a word limit which I forget) for the purpose of private study or fair criticism.
Strangely, there is no copyright on ideas, sound-alikes or look-alikes, although you could be sued (but I doubt by anyone around here) for breach of a proprietary trade-mark. This means you could release a plug-in which followed exactly the plot of another well-known game, used similar looking ships and in every way was 'almost' the real thing.
But would you want to? Goodness knows, writing EV/O plug-ins is hard enough without losing the fun of originality.
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M A R T I N T U R N E R