The Thief is Masako Tale 7 of 9, an Act 2 side tale near Kawamata Village in Kushi. You track Mai, a former Adachi servant who looted the estate after the massacre, and recover the stolen heirlooms. It rewards the Charm of Bludgeoning and 2 Silk.
Part 7 of 10 in Masako's Tales
Part 26 of 66 in Tales of Tsushima
Mai once served Clan Adachi until Masako's husband caught her stealing. Masako sent her away rather than let her be flogged. After the massacre, Mai went through the estate and took valuable family heirlooms. Masako takes this one personally, and the tale hints strongly that the two were once lovers.
Check in with Junshin, then meet Masako. Follow her until you spot Mai with a group of ronin, and hide in the pampas grass close by. The ronin are forcing Mai to take them to the stolen goods, so you shadow them all the way there, then deal with the ronin. Afterward you talk to Mai and go with her and Masako to dig up the heirlooms.
Talk to Junshin and then Masako. Go with Masako until you spot Mai. Crouch in the pampas grass near her. Shadow Mai to the hidden goods. Kill the ronin. Talk to Mai, follow the pair, dig up the heirlooms and talk to Masako.
Mai admits she felt betrayed by her banishment and still loves Masako. She never met the ringleader, only received written orders, but the notes suggest that person wants to found a new clan and is somehow related to Masako. Masako spares Mai in return for the heirlooms and lets her keep a golden comb she once gave her.
The Conspirator
The Conspirator is Masako Tale 8 of 9, an Act 2 side tale in the Kushi Grasslands where Masako turns her suspicion on Junshin. It ends with a duel against Masako herself and rewards the Charm of Dual Destruction II and the Seasons of War Armor Dye.
The Family Man
The Family Man is Masako Tale 6 of 9, an Act 2 side tale that starts at Omi Monastery in Kubara. You talk the monk Junshin into revealing where a former Adachi retainer lives, then go after him. It rewards the Charm of Ferocity and 2 Silk.
The Traitor
The Traitor is Masako Tale 4 of 9 and the last Act 1 part of her hunt, set in Ariake. You use a supply map and a monk's help to corner Sadao at a lighthouse. It rewards the Charm of Incineration, the Warrior's Bounty Dye Kit and 2 Silk.
Image: Sucker Punch Productions / PlayStationThe Tale of Lady Masako
The Tale of Lady Masako is the Act 1 story tale where you recruit Masako Adachi after her clan is wiped out. It rewards the Samurai Clan Armor and is also Masako Tale 1 of 9, opening her side storyline.