The Last Story/Shipwreck

Find the antidote
Yurick is standing in the staircase room just outside the captain's room. He says that he wants to investigate the shipwreck on the island. Tell him "I'm coming too" to go to the shipwreck. When you enter Seek Mode, look down and to the right at a watery hole in the floor to gain access to the depths of the ship. Follow Yurick through the ship and open the door he stops at. Follow the path through the door. Inside, you get ambushed by two poisonous Giant Crabs. Defeat them as quickly as possible, before their toxins get the better of you. When a crab falls off the ceiling, get close to it and press when prompted to strike the finishing blow. When both are down, follow Yurick and open the door beside where he waits for you. Through the door, Yurick collapses from the effects of the poison. You need to find an antidote, and quickly. The two rooms just ahead of where Yurick collapses contain weapons and upgrade materials. Further ahead, at the end of the corridor, climb the ladder to the next area. Look in the chest to the right to find an antidote, and hurry back to Yurick. After he's taken the antidote, climb back up the ladder. Upstairs, walk towards the curtains in front of you, where Dagran and Mirania make their entrance. Dagran then offers you a choice between the left door and the right door. Whichever door you choose makes no difference to the storyline; you'll be paired with Yurick either way, and Dagran and Mirania will take the other.

Left door
If the smell is to be believed, you've chosen the food storage. First up is another Giant Crab. This one's weak to fire, so Yurick's presence should make it a lot easier. Afterwards, enter the door Yurick goes to. This room contains several Undead warriors. Deal with the archer first, or he'll make hell for Yurick. With that taken care of, the rest should pose little danger. Take the door at the top of the small flight of stairs to rejoin Mirania and Dagran.

Right door
Through the right door, follow Yurick up through the tight corridors. At the end, you'll be prompted to enter Seek Mode. Look at the barrels in front of you to have Yurick destroy them, then follow him and enter through the hole in the wall. In this room, there's a hidden door in the back-right corner. Break through the wall with, and do the same again with the wall just through here. Follow the path to a door in front of where Yurick stops. Inside is another Giant Crab to deal with. Once it's defeated, enter Seek Mode, and look at the barrels to the left. Yurick will destroy them, opening the path for Dagran and Mirania below. Enter the door to the right to rejoin them. Head to the end of the walkway, and drop down through the gap to the left.

Finding Yurick's father
When you rejoin Dagran and Mirania, you're faced with more Giant Crabs. Take them out, but be wary of the Undead Archers on the upper level. If you use Seek Mode, you'll see some barrels behind them - destroy them to make light work of the archers. This should make it easier to take down the crabs. After the cutscene, follow Yurick, who reveals a ladder to the captain's quarters. Up here, enter the door just in front of you. Inside, ignore the appetising feast on the table, and head right, where you'll run into a mirror. Check it with, and the whole mirror moves backwards. Follow Yurick up the corridor, and through the door at the end. Save before entering the next room. Inside, Yurick's found the body of his late father. After reading part of his father's last diary entry, Yurick is interrupted by a noise from behind the curtains. Investigate to reveal what appears to be another mirror...

Boss: Doppelgänger
...but this time, it isn't what it seems. Don't try to attack the fake Zael, or he'll counter you. Instead, wait for Yurick to launch a fireball at it, then guide it into the magic circle (remember that forwards and backwards movements are mirrored by it). Repeat until it's lost half its HP. At this point, it will shatter the "mirror", and climb onto you. Shake it off by rotating. It then splits into copies of your party, identical in every way, except for the eyes. Be extra careful, though; if you aren't, you could end up taking out your own allies. Try to concentrate on their Mirania first, or they'll just heal continuously. Diffuse your Mirania's heal circles to seal their magic, and, if you want, command Yurick to Reverse their heal circles. Rinse and repeat until all four are defeated. After the battle, Yurick finally gets to learn the truth about his father's disappearance, and you return with him to the warship as true friends. For your troubles, you also get the Memento Dagger, Yurick's father's favoured weapon. It can only be equipped to Yurick, and is one of the best weapons available to him - unfortunately, this only becomes apparent in your second and later playthroughs, when it can be upgraded past +5. It's still useful in these early stages of the first playthrough, but later on, other weapons will probably outperform it.

If you haven't yet been to the Mysterious Forest, find Mirania in the room just before the stern of the ship, and speak to her to go there. If you have, upon returning to the main deck you'll be greeted by a crewman, who tells you that the door he was repairing (just before where Yurick is) is fixed. Head there to advance the storyline.