A prewar submarine reconnaissance of a Japanese anchorage

Did a Royal Navy submarine enter a Japanese anchorage before the war to photograph or inspect the fleet?

Yes; the reconnaissance gathered substantial information, although no identifying details are given.

The remembered episode of a Royal Navy submarine entering a Japanese anchorage before the war is accepted as genuine. The mission reportedly gathered considerable information, but the short exchange gives no boat, anchorage, date, or surviving report. It provides a lead for further research. The viewer's tentative identification of a P-class submarine remains unconfirmed.