When the UC3 Nautilus sailed from the Port of Copenhagen on the evening of Aug. 10, the homemade submarine bore just two people: its famed Danish inventor and the Swedish journalist reporting on his invention. Less than a day later, only one of them was still alive. Peter Madsen's submarine sank midday Aug. 11; he was rescued by a private boat, but journalist Kim Wall was nowhere to be found. It was Wall's boyfriend who reported her missing, and authorities wasted no time in arresting the 46-year-old Madsen after he came ashore, charging him that same day with manslaughter and...