To Catch a Thief

To Catch a Thief is an optional Act 2 main quest in Dragon Age II. It is only available if Isabela was recruited during Act 1.

Acquisition

This becomes available at the conclusion of Following the Qun. Return to the Hawke Estate in Hightown to find Isabela and Aveline arguing, both in need of help. Hawke must choose who to help first.

Note: If Hawke does not choose to help Isabela first, she will leave the party permanently.

Walkthrough

Isabela reveals that she found the relic that she had lost. Her pursuer, Castillon, will spare her life if the relic is returned.

Head to Lowtown with Isabela in the party to find Wall-Eyed Sam. Things begin to heat up with a fight against a Sten and a couple of Qunari. A cutscene reveals that the relic Isabela had stolen was a tome written by Ashkaari Koslun, the founder of Qun. The Qunari have been unable to leave Kirkwall without the relic. At the end of this conversation, Hawke chooses between helping Isabela, which gives:

Isabela: friendship (+15),
Varric: friendship (+10),
Merrill: friendship (+10),
Aveline: friendship (+5)
Sebastian: rivalry (+5),

Or returning the relic to the Qunari, which gives:

Isabela: rivalry (+15),
Merrill: rivalry (+10),
Aveline: rivalry (+5),
Sebastian: friendship (+5).

Enter the Dark Foundry for a fight with Tevinter Mages and some Qunari led by a Sten. Isabela runs away at the start of the battle to chase Wall-Eyed Sam, leaving Hawke and two other party members to plow through the remaining enemies (assuming Hawke has kept his/her distance until the Qunari or the Tevinter Mages have killed each other). In the room to the north-west, you'll find Lambswool Insoles in a crate - an armor upgrade for Isabela.

Note: The saw blade shut off switches are Master level, meaning they require 40 cunning to detect (or the equivalent using bonus to disarm traps items). No XP is received for pulling the levers. There are two in total, one through each door on the long single room to the north east of the area.

After leaving the building, Hawke will find Isabela's Letter on Wall-Eyed Sam's body and the quest ends.

Result

Isabela's letter contains an apology and says that she is leaving. If Isabela is romanced or has a Friendship/Rivalry score of fifty or greater, she will return with the relic during the quest Demands of The Qun.

Rewards

1000 XP (quest completion)
Several generic Qunari-related weapons, accessories, or armor
Non-generic rewards:

Gauntlets of the Ashaad

Lambswool Insoles (Isabela's armor upgrade)

Trivia

While Isabela is describing the relic she mentions one of the options as Cousland, which is the family name of the human noble Grey Warden in Dragon Age: Origins.
The name of this quest is likely a reference to the 1955 movie To Catch a Thief

Bugs

There is a chance that Hawke won't have to battle the Sten and his Qunari at all. Just move straight towards the foundry entrance and click on it. Hawke will be brought to a cutscene without having to fight.