I am interested in writing a novel about a song from a band. It will have the exact same plot but different names of the characters. Is this considered plagiarism? The idea of the song and my book are exactly the same.
Ideas are not protected by copyright. What is protected is the "unique tangible expression of that idea", in this case the words and music of the song. A derivative work does not infringe on copyright if it is not substantially similar to the original.
I don't know how a court of law would rule, but to me personally a book that retells the story in a song is substantially different enough not to infringe the copyright of the original, even if it did use the same character names.
There are many songs that retell a work of literature, so I would think the reverse must be legal as well.
Don't forget about trademark law! Disney can't copyright the idea of a talking mouse, but you will get sued for infringement if you appropriate their trademark, which could be an image of a mouse that has white gloves or a mouse that is named Mickey, for example.
Remember that Apple Records and Apple Computer had a tussle over trademarks.
So, while you may be safe in using ideas from a song, you might be infringing on trademarks if you use specific names or phrases. Although, lots of trademarks are not as common among bands as they are in commercial companies.
Trademark law can be as tricky as copyright. You an find a summary at the the Unites States Patent and Trademark office: