HMS PARTHIAN (Lt C A Pardoe) sailed from Malta on 22 July 1943 for her 21st war patrol in the Southern Adriatic. PARTHIAN was instructed to patrol off the West coast of Greece North of the 39th parallel. On 26 July, the orders were changed to patrol 10 miles East of Cape Otranto and, on the morning of 28 July changed again to patrol between 40°N and 41°30’N in the Adriatic as it had been reported that Italian battleships were exercising in that area. On 30 July, the Captain SM10 ordered PARTHIAN to stay clear of areas where two other submarines were operating.

On 6 August, the submarine was ordered to leave her patrol area and to complete her patrol at Beirut in the Lebanon on 11 August. HMS PARTHIAN failed to arrive as expected, was declared overdue and later as lost with all hands, probably having struck and detonated a mine. Four officers and sixty-one ratings were lost. The wreck of the submarine has still not been located.