The Mamutu was never built for heroics. Constructed in Hong Kong in 1938, she was a modest vessel of 300 gross tons, 113 feet long with a 25-foot beam, designed to haul cargo between the islands of New Guinea for Burns Philp & Company. But when war reached the Pacific, modest ships took on urgent purposes. Through 1941, Mamutu served as a Royal Australian Navy stores ship, ferrying supplies to scattered outposts. By early 1942, with Japanese forces advancing through New Guinea, she was pressed into a more desperate role: evacuating civilians ahead of the invasion. The people aboard that August day were fleeing for their lives, trusting the little merchant vessel to carry them to safety.

The submarine that found her was Ro-33, a Japanese boat under Lieutenant Commander Shigeshi Kuriyama. With a surface speed of 19 knots, she closed the distance quickly. At 300 meters, Kuriyama gave the order to fire the 3-inch gun mounted forward of the conning tower. The first shell destroyed the radio room, killing wireless operator Furbank. The second carried away the bridge and Captain J. McEachern with it. More shells tore into the hull, and within minutes the Mamutu was a shattered wreck, her decks strewn with the dead and dying. Then Kuriyama ordered something worse: his gunners opened fire with 13mm machine guns on survivors struggling in the water. Men, women, and children. He withdrew only when there was no one left to shoot at, leaving the Mamutu to sink with most of her complement dead.

Of 120 people aboard, only 28 survived. Billy Griffin, who had escaped the machine gunners by playing dead, would prove essential to a second set of survivors as well. When a Royal Australian Air Force rescue plane, the Calypso, crashed during the rescue attempt, killing one of its twelve crew members, Griffin helped the surviving airmen reach land and eventually make it back to Port Moresby. A United States Army Air Forces B-17 Flying Fortress dropped life rafts to other survivors struggling in the water. The Australian Army signal ship MV Reliance, dispatched from Murray Island to search for survivors, never found them. Those who lived drifted on air-delivered rafts until they reached shore. Griffin's heroism was formally recognized eight decades later, at a commemoration service on April 30, 2022, where his name and those of the Calypso crew were engraved on a memorial walk at North Head, Manly.

Justice, when it came, was swift and absolute. Three weeks after sinking the Mamutu, Ro-33 torpedoed another Burns Philp vessel, the 3,310-ton Malaita, as she left Port Moresby under escort by the Australian destroyer Arunta. While the Malaita was towed back to port, Arunta's crew made ASDIC contact on the submarine. A series of depth-charge attacks followed, and Ro-33 went to the bottom ten nautical miles southeast of Port Moresby with the loss of all hands. Kuriyama and his crew would never answer for what they did to the people on the Mamutu, but neither would they harm anyone again. Today, both vessels rest on the floor of the waters around Papua New Guinea, silent reminders of a war that spared no one, not even civilians fleeing on a small merchant ship.