It depends entirely on the situation. It all comes down to whether you have permission or not, be it to make an adaption at all, to make it a certain way, etc. A work being in a different medium doesn't automatically exempt it from charges of plagiarism seeing as 'plagiarism' isn't just 'copy and paste'. Plagiarism according to OED: "the wrongful appropriation or purloining, and publication as one's own, of the ideas, or the expression of the ideas [...] of another."