Millicent's questline starts in Caelid with Gowry and later travels through Altus Plateau, the Mountaintops and the Haligtree.
Checkpoint 1: Gowry and the needle
Speak with Gowry at his shack, then obtain the Unalloyed Gold Needle from Commander O'Neil and return it so the quest can continue.
Checkpoint 2: Church of the Plague
Take the repaired needle to Millicent at the Church of the Plague and continue her dialogue until the Caelid section is resolved.
Checkpoint 3: Valkyrie's Prosthesis
The Shaded Castle contains the Valkyrie's Prosthesis, an important item for advancing Millicent's route when she appears in Altus Plateau.
Checkpoint 4: Later regions
Continue following Millicent through later locations, including Windmill Village, the Mountaintops and eventually the Haligtree.
Final choice
Near the end, the quest presents a branching choice involving whether to assist or challenge Millicent. Treat this as a deliberate decision because the rewards differ.
Why a separate tracker helps
This quest is spread across distant areas. Record the last location and whether the prosthesis or needle step is complete before moving on to another storyline.
Reference: GameSpot Millicent quest guide.