Marche gathers mythril and is promptly accused of stealing it. Sure enough, Marche's ploy to get Doned's attention has worked.
You must fight a Juggler, a Sage, an Animist, a Blue Mage, a Mog Knight, and a White Monk.
The Juggler has some status-affecting skills and generally has high attack power.
The Sage has Giga Flare, which is immensely powerful. Keep your units apart from each other to avoid having more than one of them get hit in a turn.
Also, the White Monk has Earth Render, so be careful not to put your units in a line. You might want to go for the White Monk first because he has Revive, which can make the battle drag on for a long time.
The Blue Mage can do Roulette, which will randomly KO a unit (even an enemy unit). This can get pretty annoying, so the Blue Mage should be relatively high on your priority list. He can heal himself, too, and he has Bad Breath, which can throw your units into chaos with things like Frogshape and charm.
There's also a Mog Knight. He can heal himself, but his other skills aren't too dangerous.
If you use a law card to prohibit Skills, then the Sage can't do anything. Also, if you use a law card to prohibit Color Magic, that renders the Blue Mage powerless.
Once you defeat these foes, Marche calls to Doned, telling him to come out from hiding. Doned realizes that this was just a trick on Marche's part so that Marche could try again to convince Doned to return to the real world. Doned is angry, saying that he is able to do whatever he wants in this world, and Marche just wants to take that away and make Doned lose everything. But Marche tells him that since his mother had to raise the two of them by herself, Doned always got all of the attention and Marche had to sacrifice all of his desires for Doned's sake, which proves that Marche doesn't have everything that he wants in the real world. Marche has felt alone since he and Doned and their mother moved to St. Ivalice.
Marche believes that even if they go back to the real world, Doned won't have to lose what he's gained in this world. He believes that Doned will be able to run again in the real world, and Marche promises to do whatever he can to help.
With that, Doned realizes that going back to the real world will be worthwhile after all, and he and Marche make up.
Once you complete Hidden Vein, you can place the Deti Plains symbol on the map. The next time you go to a city symbol, a cutscene will take place. Doned is there with you, and Marche receives a letter from Judgemaster Cid, who says that he and Babus have discovered Mewt's location: in Ambervale with Queen Remedi. Cid asks you to meet him on Deti Plains.