By the summer of 1944, the Imperial Japanese Navy’s I-29 was no ordinary submarine. She was fast, long-ranged, and highly prized, not for the number of ships she had sunk, but for the secrets she carried. She had just returned from Nazi-occupied France, the only Japanese submarine to survive the perilous transoceanic “Yanagi” missions under the Tripartite Pact. These voyages were the Axis powers’ last hope at exchanging technology and critical war materials across submarine lanes, now that surface traffic had become suicidal.