Overview
The Bitter Pill is a Doctor Galen side quest that surfaces after I Need a Bigger Boat. Galen suspects a traitor inside the Governor's inner circle is leaking information to Blackbeard. The quest asks the player to gather evidence from pirate-held sites and confront the traitor, Francois Arnaud, at Tortuga.
How to Start
After completing I Need a Bigger Boat, speak to Doctor Galen at the base and choose the dialogue option "Is something troubling you, Doctor?" to trigger the quest.
Walkthrough
Talk to Galen. He explains his suspicion and directs the player toward Thomas Richards as the starting thread.
Follow markers from the Revenge Is Best Served Cold quest and collect Black Marks from four chests scattered across the Coastal Jungle.
Return to Galen with Francois Arnaud's letter addressed to Blackbeard. Galen asks for a final confirmation.
Sail to the Blackbeard Ruins Excavation Site (southwest of Richards' camp). At the northwest end of the ruins, find the Gunpowder Crates tucked against a steel cage.
Return to Galen one more time, then travel to Tortuga.
Dock at Tortuga and take the right branch at the first intersection. Confront Francois Arnaud with the evidence.
Francois reveals that Blackbeard has kidnapped his son Jacques, forcing him to spy. The quest concludes with the confrontation.
Rewards
The Bitter Pill's rewards are story progression and additional dialogue with Galen that unlocks later threads. Loot from the intervening Black Mark and Excavation Site steps contributes to the Revenge Is Best Served Cold payoff (Galen Sword, faction XP, etc.).
Tips
Because The Bitter Pill overlaps with Revenge Is Best Served Cold, complete both in parallel to avoid re-sailing the same routes.
Bring a full loadout: the Black Mark chests are defended by Blackbeard patrols.
Do not attack Francois. The quest resolves through dialogue, not combat.