'Z' (iii) was the Japanese initial plan for the defence of the Mariana islands group against US invasion by the interception and destruction of the US Navy’s invasion force as it approached the islands (spring 1944).
The plan was developed by Admiral Mineichi Koga, commander of the Combined Fleet, and then revised into the definitive 'A' by Admiral Soemu Toyoda, who succeeded Koga after the latter’s death on 31 March 1944.
The plan represented the first stage in the development of the scheme that was the Imperial Japanese navy’s last attempt to bring about the Kantai Kessen, the decisive battle which would destroy Admiral Chester W. Nimitz’s Pacific Fleet and thereby reopen the way to victory for Japan. The 'Z' (iii) plan was approved by the Imperial Japanese navy general staff with the title Combined Fleet Secret Operations Order No. 73, and the definitive version was issued on 8 March 1944.
On 31 March, while flying to Davao in the Philippine islands group, Koga was killed when the Kawanishi H8K 'Emily' flying boat in which he was travelling crashed into the sea during a storm. Koga’s chief-of-staff, Vice Admiral Shigeru Fukudome, carrying a copy of the plan, was flying the same route in another aeroplane, and this crashed near Cebu island in the Philippine islands group after attempting to avoid the same storm. Fukudome and nine others eventually made it to shore, where they were captured by Filipino guerrillas. As a result of the loss of these two aircraft, with their important passengers and documents, the Japanese applied intensive pressure on the local population, and Fukudome and the others were eventually released.
On 3 April a wooden box from Fukudome’s aeroplane was washed onto the coast and recovered by local villagers. The box contained the 'Z' (iii) plan, and this eventually reached the local guerrilla organisation, the Cebu Area Command, under the leadership of Lieutenant Colonel James M. Cushing. The ornate nature of the bound document led to the belief that the package was very important, a suspicion that was later reinforced by Japan’s offer of a large reward for the return of any documents. A secret submarine pick-up was arranged, with a cover story of evacuating US refugees. The submarine picked up the documents, along with 40 US men, women and children. Travelling mostly on the surface for speed, and diving only when needed, the submarine survived depth charging twice, arriving near the naval base at Darwin in northern Australia on 19 May. From there the documents were flown to Brisbane.
The 'Z' (iii) documents were in plain text rather than code, and were quickly translated by the five best translaters at the Military Intelligence Service attached to the Allied Translator and Interpreter Section. Copies of the translation were rushed to General Douglas MacArthur, commander-in-chief of the South-West Pacific Area command, who speedily forwarded them to Nimitz, commander-in-chief of the Pacific Ocean Areas command and of the Pacific Fleet. Among other things, the important intelligence coup meant that planned Japanese diversionary tactics could now anticipated by the Americans, leading to the US victory in the Battle of the Philippine Sea during 'A'.