The Australians make a large appearance here, as your mission is to infiltrate Sydney and torpedo an ammunition ship.
- Sneak into Sydney Harbour following the waypoints.
- Identify the four indicated cargo ships.
- Sink the indicated cargo ship.
- Do not raise the alarm.
- Locate the downed Japanese recon plane.
This mission is extremely straightforward. The only thing you will have in this mission is a single Japanese Type A midget submarine. Your first task is to slip into the harbour undetected - something that is easier done than said, in fact. The only way to trip the alarm is to surface - being in range of enemy sonar, striking Sydney's anti-submarine boom net (which is basically an underwater wall), and even torpedoing the cargo ships won't set off the alarm. You can even torpedo the destroyers in the harbour and the alarm still won't trigger.
The Australians have a lot of patrol boats though, so if you dawdle you'll run out of air and be forced to surface, tripping the alarm in the process. If you do trigger the alarm, the best you can do is try to torpedo the destroyers to blind the PT boats - or simply restart.
Out of the three major threats in the mission - the boom net, the mines, and the patrols - the deadliest thing is the mines. They are proximity fused, so avoid them at all costs. Losing all three minisubs will result in a game over.
When you start the mission, you begin close to a cargo ship. Your superior suggests moving underneath it to slip in unnoticed, but you don't really need it. The net and the mines leave a very straightforward path to follow. If this is your first playthrough, though, you may want to follow the cargo ship and have it pave the way for you - it will run into neither mines nor the submarine net. Avoid the surface and you'll be fine.
There will be four waypoints for you to follow. The whole way, feel free to torpedo anything you want - just save two torpedoes for the mission critical target at the end. Try not to dawdle though, as your oxygen supply is limited - more so than a standard fleet submarine. Once you've passed the fourth waypoint, you'll hit a checkpoint and have passed the alarm.
Once you're inside, you can surface and do anything you want in Sydney. Before proceeding to identify the listed cargo ships, though, turn around to the way you came. Pull up your tactical map and search for a grey dot labelled O-815. Head right for it, and as you close in you'll see that it is a downed Mavis. Move in closer to trigger some dialogue and complete the mission's hidden objective.
Next, bring up your tactical map again and search for a long, winding river system. Near the north, you'll see a large chunk of brown on the west side of the river. Follow the river and head for this area. Once there, you'll have located a large cave with a large Rising Sun billboard. Locating this cave yields the Underwater Hideout achievement.
With the secret objectives out of the way, proceed back into the harbour proper. The cargo ships you must search are located in a fairly straightforward line for you. From east to west, the HMAS Westralia, HMAS Adelaide, USS Dobbin, and the HMAS Kanimbla. Bring your submarine close to them to search them. Search them in any order you wish, but it is preferred to scan them in the order listed above.
Upon scanning the Kanimbla, your superiors will note that they've found your target. Note that your superiors will always note that the first ship you scan contains "civil cargo" - the only way to know which ship to torpedo is for your superiors to tell you once you've scanned all four ships. Torpedoing one of these ships before you've scanned it results in a game over screen.
Torpedo the designated cargo ship to complete the mission.