At the end of ‘The Almost People’ it is revealed that Amy had actually been replaced by a ganger since before she re-joined the Doctor in ‘The Impossible Astronaut’. Unknowingly controlled remotely by the real Amy the only clue was occasional appearance of Lady Kovarian, peering at her through a hatch.
The world of Doctor Who allows several variations of this, if a games master wishes to try something similar in their own game. This can be done to create an on going plot arc and is a great way to end an adventure on a cliff hanger.
Just as it was used here the player characters have seemly saved the day only to make the startling revelation that one of them isn’t who they appear to be. This can build up the anticipation for the next game, where everyone comes to the rescue.
There are two approaches you can take here. You can discuss the idea with the player whose character you wish to replace. This can allow you to co-operate and is particularly suited to players that you can trust and who will appreciate the challenge.
Secondly you can keep them in the dark. This works best for duplicates who aren’t aware of their true nature. They might realise that something is different with their character but not be sure if they should tell the others.
Next you can decide the nature of the cuckoo in their midst. Gangers aren’t the only way to replicate a character. Their stand in could be a robot, either one that is made to look and act like the character or one that can shape change. Both Kamelion and the Teselecta would fall into the latter category.
There are also several species that specialise in impersonation. The Autons, the Family of Blood (or at least their species), Rutans, Zolfa-Thuran and Zygons all could take a player character’s place. Even the Slitheen could conceivably make a flesh-suit that looked like a particular person.
The question will be whether the impersonator knows that they aren’t the real article. Some could be conditioned to think they are the person they’re replacing, allowing a player to continue portraying the character as normal.
There may just be moments when their secret programming kicks in, resulting in brief memory gaps. They might also find that medical scans give some very odd results. If they aren’t organic they could find that they don’t need to eat or sleep. An injury might not produce blood.
Alternatively the impersonator could know exactly what they are. In this case the player might wish to act a little differently, although not enough to tip of the other players. They might also look for opportunities to carry out their agenda, away from the prying eyes of their companions.
Which brings us to why the switch has occurred. The Silence wanted Amy in order to obtain Melody Pond. They didn’t want the Doctor to realise she was missing to prevent him from rescuing her before she gave birth.
Their plan is even more clever when you realise that by having Amy synched up to the ganger in the TARDIS the Doctor couldn’t rescue her without risking a paradox. If he did then all her actions as the ganger would be erased, causing no end of problems for the Doctor.
In other scenarios the impersonator might just want to get close to the player characters, particularly if they have access to a TARDIS. They could bide their time, learning how to pilot the vessel themselves, waiting for an opportunity to steal it.
They might want resources which the TARDIS can take them too. They can subtly manipulate the pilot of the vessel, perhaps asking if they can visit times and places that will advance their plans. When their true identity is revealed you can also show what they have gathered from past adventures.
The impersonator might have a noble mission. They could searching for a cure to a disease or a solution to a problem troubling their place of origin. They could be seeking revenge or engage in a crusade against a particular alien race.
The most direct mission would be to kill the player characters. The idea would be that direct attacks rarely work so the impersonator will try to destroy them from the inside. Player characters could find themselves beset by misfortune, only to discover they are being sabotaged by someone in the group (maybe without them even knowing).
On the other end of the scale the purpose of the swap could be to obtain the character being replaced. The fact that their companions are time travellers is a complete coincidence or at least incidental.
In this scenario the person being held must have some unique quality that is vital to the kidnappers plan. This does open the possibilities of running parallel adventures, with the kidnapped player character having to carry out tasks for the kidnappers while their companions travel through time with the replacement.
A reoccurring plot development in this kind of story is the impersonator having a change of heart. Being treated like the person they’re replacing can convince them that the player characters are good people. They might even turn against whoever they were working for.
This can lead to the duplicate making a heroic sacrifice to save the original. The idea being that the kidnapped person is so noble that anyone who pretends to be them will gain that same nobility.
Duplicates are also a great escape clause should a player character be killed. Just as their companions are about to mourn they see that the dead body is a robot or transforms into their original alien form.
Once the rest of the player characters have discovered the identity of the duplicate they must then plan how to recover their missing friend. Where and when are they? Who has them and why?
All of which leads us to our next topic, the rescue.