The submarine departed Australia on 20 March on her last war patrol and headed for the Strait of Malacca, gateway between the Pacific and Indian Oceans. Patrolling along the Malay and Thai coasts, "Grenadier" claimed a small freighter off the island of Phuket on 6 April. She remained in the area and late in the night of 20 April sighted two merchantmen and closed in for the attack. Running on the surface at dawn 21 April, "Grenadier" spotted, and was simultaneously spotted by, a Japanese plane. The submarine dived, and as it passed her executive officer commented, "We ought to be safe now." Just then, explosions rocked "Grenadier" and heeled her over 15 to 20 degrees. Power and lights failed completely and the fatally wounded ship settled to the bottom at . She tried to make repairs, while a fierce fire blazed in the maneuvering room.