‘Time Warrior’, by Robert Holmes, introduces Sarah Jane Smith. When the Doctor first meets her she is posing as her aunt, to infiltrate the gathering of scientists. The Doctor quickly sees through her ruse and she confesses that she is a journalist. Professor Rubeish worries that she might be a spy.
While it is ultimately revealed that she is indeed a journalist how does the Doctor know that for sure? She would hardly confess if she really was a spy and the Doctor already knows that scientists are going missing.
During this UNIT era there was a concern about foreign powers abducting (or luring away) scientists. The loss of individuals with unique skills or exceptional ideas could shift the balance of power, forever determining whether capitalism or communism was victorious.
The Brigadier is therefore right to be worried, even if putting all of his eggs in one basket might not have been the best idea. The Doctor, however, seems unconcerned by the mystery of the missing scientists.
While it is true he doesn’t have an allegiance to one particular country (being a Time Lord) he does have some loyalty to UNIT and must realise how it would be affected if the East gained the upper hand.
It is possible that his knowledge of future history allowed him to be confident that it wasn’t a foreign agency taking the scientists. It is is also possible that with control of the TARDIS returned to him if things became uncomfortable in the UK he could simply leave. How might things have been different if he was still in exile and would have to live with the consequences of political upheaval?
Responding to Rubeish’s question the Doctor puckishly suggests that they shoot Sarah Jane, if they she is a spy. Rubeish doesn’t take this seriously but confirms that there is something odd about her and that she even tried to convince him that the Doctor was a spy.
This illustrates that Sarah Jane could have an antagonist in this story. Had it not been for the main plot about a Sontaran in the middle ages using time travel to kidnap scientists the whole story could have centred on the Doctor matching wits with a potential spy, even being framed.
We can presume that Sarah Jane made these allocations to throw doubt on the Doctor, thus colouring any claims he might make against her. This is quite a ruthless act for a young journalist. This is a side of Sarah Jane that we rarely see later.
Trust has to be earned and early on in a relationship things can be tense. It can take a while to work out who a person really is and what their agenda is. When the stakes are high people can be reluctant to put their faith in others.
This can be important during the early sections of an adventure or campaign. Not only could the PCs be trying to work out whether they can trust other PCs they also need to know which NPCs they can trust.
The presence of spies, who lie as part of their job, make this more complicated. Suspicion and paranoia can be rife in games in which espionage have some presence. This can add levels of complexity to an otherwise straightforward story.
This could be the main focus of a UNIT campaign. They have secrets they need to protect and certain missions could give them an opportunity to learn more about what their enemy currently knows.
Espionage and spies can occur in any setting and time period. Whether it be behind enemy lines in WWII, during the Cold War, in the corporate run future or in the stars with multiple alien empires.
Shape shifting aliens and brain washing techniques (such as those favoured by the Daleks) mean that the PCs can never fully trust anyone, not even themselves. This can result in exciting twists in the plot, where characters reveal their true allegiances.
Just the suspicion of a spy can cause problems for PCs. As time travellers with few credentials their odd behaviour and unexplained motives can mean that the authorities have reason to arrest, interrogate and even execute them.
This can add another task for the PCs to complete. Not only must they stop whatever threat exists but they must prove their innocence and identify who the real spy is (if there even is one).
It should be remember that a spy is not automatically a bad person. Thus you might want to have characters who aren’t what they really seem who end up helping the PCs. For example if a PC is captured by the enemy maybe one of the guards is a spy who helps them escape.
Exploring this idea you could reveal that other characters were always undercover spies. For example Sarah Jane Smith might actually have been a spy (using journalism as a cover) which could put a new twist on adventures set during her travels with the Doctor.
Who might she be working for? What secrets would she have access to once the Doctor gets her UNIT clearance? Would her friendship with the Doctor give her doubts? If her identity was discover how would UNIT react to this breach of security and those responsible for it?
This could allow one or more adventures revealing this untold chapter of Sarah Jane’s life. It could be that she eventually does become a journalist but you still have scope for some covert missions (maybe some that occur in the background of a televised story).