Index: ChapterX.Application/Chapter/Commands/UpdateHandler.cs
===================================================================
--- ChapterX.Application/Chapter/Commands/UpdateHandler.cs	(revision 877c13c3dd0934c1d31da7988d0d2c2c5925ed55)
+++ ChapterX.Application/Chapter/Commands/UpdateHandler.cs	(revision a6e33d1f34c0dbbfff38bfb2c73eea32abfe7cff)
@@ -18,7 +18,10 @@
         public async Task<UpdateResponse> Handle(UpdateRequest request, CancellationToken cancellationToken)
         {
-            var chapter = await _chapterRepository.GetByIdAsync(request.Id, cancellationToken);
+            var chapter = await _chapterRepository.GetByIdWithStoryAsync(request.Id, cancellationToken);
             if (chapter is null)
                 return new UpdateResponse(false);
+
+            if (chapter.Story.UserId != request.CallerId)
+                throw new UnauthorizedAccessException("You do not own this chapter.");
 
             chapter.Number = request.Number;