The couple at the table (Tapa blanda)

You're on your honeymoon at an exclusive couples-only resort. You receive a note warning you to 'Beware of the couple at the table nearest to yours'. You also don't know that you're about to be murdered, or that once you're dead, all the evidence will suggest that no one there that night could possibly have committed the crime. So who might be trying to warn you? And who might be about to commit the perfect impossible murder?