Talked with one player ahead of time (archer) regarding the doppleganger switch. Then, in front of the rest of the group, played out having that PC get jumped in an alley. The player went along perfectly and "returned" to the party to tell his story. Of course, the PC lost the encounter but we played it out as if he'd escaped - i.e., the doppleganger's cover story. The rest of the group went along with the scenario because they had just listened to the entire "encounter."
Doppelganger decided to betray the party when they were split up in the plank room. The monk was injured and had to flee into the well, trying to escape - doppelganger in pursuit.
The wizard fell back through the warehouse entry, going to retrieve the fighter, who had stayed on guard outside (primarily because that player couldn't make the game). I announced he'd made it outside.
That's where we stopped.
Before the next game, I plan on hooking up with the wizard and truly finishing off his pursuit. If he fails to elude the pursuing doppelgangers (from the cell), one of them will take his form and burst outside to lure the fighter inside, using the same method as the archer.
This is working great. Much better than imposing saves, hints, commands to the "controlled" PC during the game. The party is confused and paranoid. According to the monk - "The saying goes keep your friends close and your enemies closer but I can't tell which is which (witch?)".
Great job, Katana! Sounds like you players are really enjoying the module. Pulling off the switch is very tough to do.
We have a paladin in the party. The doppelganger that replaced the party member was Neutral (this helped keep him under the paladin's "detect evil" radar). The doppelganger was hired to infiltrate the party and spy on them, not to destroy them.
By having the doppelganger backstab the group, you're missing out on a classic moment: when the party finds "themselves" under Sodden Hold. The doppelganger should act surprised, and the party will start pointing fingers at one another. How many people are real? How many are doppelgangers?
Of course this is D&D so you don't want to play this out too long. After a couple of minutes of name calling I had everyone roll sense motive checks. If they failed vs a corresponding bluff check then they were "surprised" when the doppelgangers attacked.