Do you have any anti-malware tools installed? If so, remove that stuff, and test again. Those tools are common triggers for weird behavior.
Use View > Message > Raw Source to see if the message is really empty.
Figure out if it's one sender, or if it's multiple senders.
Configure and try a different mail server, and see if it's something with your original provider, or something specific to OS X client.
AFAIK, Mail.app doesn't have (supported) plugins, and some of the unsupported plugins can occasionally cause issues. If you have any mail-related plugins loaded, remove them and test again.