Not sure whether this would work or not, but it might be good if enough dedicated players get involved.

Basically it would be a [i]Fallout[/i] RP, where a small settlement with a range of characters playing the everyday lives of the people struggling to survive life in the Wasteland. The wrinkle for the story would be that at least one of the characters would have been replaced by a Synth at some point, without their knowledge.

To start with all the characters would have to maintain their everyday lives, until the point came where the GM would choose one to have been replaced, and would PM that player to let them know the change in the status quo. From that point on the players would be hunting for the Synth, trying to discover which of the other players is acting differently than usual as the DM introduces new events to try and force the Synth player's hand, revealing themselves to the others. For instance the GM might give the Synth insomnia, causing them to wander around the settlement at night, or plague them with strange dreams of a previous life underground at the Institute before they were rescused by the Railroad.

Exactly what happens when the characters discover the replacement would be up to the players in-chracter. Would they turn on the fraud, accept them, or do something entirely unexpected?