The Doctors need to get in the Black Archive but it is TARDIS proof. How do they do it? In ‘The Day of the Doctor’ the War Doctor realises they don’t actually need to land. They just need the ‘Gallifrey Falls’ painting to be in there so they can emerge from it, just as the Zygons did in the National Gallery.
In order to make sure that it is there they contact McGilliop by telephone and ask him to move it there, an example of the Doctor crossing over his own timeline.
Time travel fiction frequently does this, showing an event and then revealing that it was the characters future self that was responsible. The film ‘Time Crimes’ is probably the best example of this.
Doctor Who does it much less frequently. ‘The Big Bang’ has the most examples, with the future Doctor engineering his own escape from the Pandorica and again crossing his own path later so he can pretend to die and get into the Pandorica again when everyone is distracted. This was shown to be a special situation due to the collapse of time.
It could be that the presence of three Doctors created similar conditions but such actions are probably achievable normally.
The important thing is why the Doctor does it. He has no other option but to go back in time. Similarly PCs should only resort to this use of time travel if they simply run out of time in the ‘present’.
For example the PCs might have tried their best to counter a threat (and may have succeeded in stopping part of it) only to find out there was something they didn’t know about or that the there is a backup plan. They will not now have time to prevent the threat from happening.
Note that the Doctor didn’t change anything by his actions. We had already seen the phone conversation earlier in the episode. Everything that we saw happened still happened the same way. We just didn’t know that some time between that conversation and the Zygon attack McGilliop arranged for the transport of ‘Gallifrey Falls’.
If you do allow this type of action in your own adventures it should only be possible if the PCs don’t affect their own past or present. They can only use it to prevent something that happened, not change it.
The Doctors have the advantage of already being in their TARDIS when they decide they need to travel into the past (before the countdown reaches 0). If the PCs aren’t in their own time machine they will need to race to get there before the event they are trying to prevent occurs (or at least ensure they don’t know that it has happened).
This condition can require them to use stealth and can lead to amusing incidents where they are trying to keep outside the awareness of their previous incarnations. This is best done if they are interacting events that previous happened off screen. As long as nothing happens to contradict anything that happened on screen then the plot will still make sense.
In this way the PCs can revisit the events of the same adventure. To those around them the time travellers appear as if they are in multiple places at once. ‘Back To The Future 2’ has some good examples of how to revisit the same story but from a new perspective.
The PCs might be revealed to be responsible for things they experienced the first time round. The person an NPC is speaking to on the phone is their future selves, they are responsible for parking a car across a road to prevent the bad guys escaping or they are the ones who will sabotage the aliens doomsday weapon.
PCs could spend plot point to make this happen, making a note that they need to ensure this happens by travelling into their past. Taking to far and this could turn into the farce of ‘Doctor Who and the Curse of Fatal Death’.
Such occurrences could arise naturally during play, with coincidence or luck affecting the narrative. The PCs don’t need to make sure they happen but if they find themselves in the past they could be revealed as being the source. The games master might even give a plot point reward for help close these plot holes.
When things go wrong the PCs might change their own past, or worse meet their earlier incarnations. There are ways around this, for example allowing history to be re-written, having one incarnation vanish (as in ‘Father’s Day’) or simply having the incarnations forget the encounter.
Something to avoid is the PCs creating a paradox. That is they shouldn’t bring something into the past to give to their earlier incarnations to eventually bring back into the past to give their younger self, whether that something be information or an item. In both cases it comes from nothing, existing only because of the loop.
Things get very complex when this type of time travel is used, which can be part of the fun. Care should be taken to make sure everyone keeps track of where various incarnations are and what actually happened.
What it can do is to give an adventure more scope. Rather than just one run through the PCs have to make two or more passes to ensure that everything is resolved to their satisfaction. Only a time traveller could achieve what they’ve done. This can make an adventure and their actions in it all the more special.
You don’t need to do this immediately either. You might return to the same adventure much later, revealing that there was a whole other side that they didn’t know about until know.
While this can complicate things further, as the players might not have clear memory of what happened the first time, it could be an excuse to revisit earlier versions of a character, especially if the Time Lord PC has already regenerated.
For a good Doctor Who example of the Doctor crossing his own timeline read ‘The Festival of Death’. The audio ‘Flip Flop’ explores a similar idea with its own twist with the 7th Doctor experiencing events he has put in motion, although not the way you would think.