In ‘The Snowmen’ the Doctor makes use of a Memory Worm. He apparently keeps this on hand to remove witnesses memories of anything they might have seen including their encounters with him. Clara only avoids this fate because doing so would remove the knowledge she needs to protect her from the Snowmen.
We learn precious little about the Memory Worm. All we know is that merely touching it can wipe roughly an hour of memory but if it bites you it can consume decades, as evidenced by its encounter with Dr Simeon.
The fact that touch alone is enough to remove a memory could indicate that either it can absorb memories through touch, that it secretes a substance that performs this function or that it exerts a close range telepathic connection.
It is not even clear that the Doctor is responsible for bringing it to Earth. While this is certainly possible it could also be a native to Earth discovered by Lady Vastra and her associates. It would only appear to be alien because everyone forgets their encounter with it.
If the Memory Worm is from an alien planet are they compose only a small portion of the ecosystem or are they the dominant species? Do they need to consume food or are they able to survive on memories alone? It could be that their memory erasing abilities are chemically induced, a way to prevent predators even been aware of their presence.
The Memory Worm presents a good basis for a Mystery adventure. Missing memories and time can lead to confusion amongst colonists or natives of the planet that are home to the Memory Worm. The PCs might be required to discover what is happening and find some defence against the worms.
Rather than a menace the Memory Worm can act as a useful tool amongst a variety of species. The Cybermen could make use of it to wipe a subjects mind, prior to conversion. Without memories emotions would be easier to remove. A warlike species could wipe out the memory of their atrocities from other species memories or those who use infiltration, such as Zygons, could use memory gaps to cover their secret invasion.
The Doctor demonstrates how the ‘good guys’ can also use the Memory Worm. Here the Doctor is keeping a low profile and doesn’t want anyone forming an attachment to him to drag him out of his self-imposed retirement.
PCs who wish to operate covertly could use the Memory Worm to cover their tracks at the end of each adventure. This is especially appropriate if the PCs are being hunted down (for example by the Time Lords).
Rather than building legends through their adventure they act as shadows, operating behind the scenes. The price of anonymity is that they don’t gain a reputation. Even if they saved the world a hundred times those in authority wouldn’t trust them.
Secret organisations, such as Torchwood and UNIT, could also find the Memory Worm useful. Indeed studying the worm might be how Torchwood develops their own amnesia inducing chemicals.
Pre-Time War the Time Lords might have used the Memory Worms when exiling prisoners. Rather than deleting memories it just represses them, allowing them to temporarily remove knowledge of how to use a TARDIS.
If there was specific incident that spurred a Time Lord going rogue then memory erasure could return them to the fold. An adventure could be based around the Doctor (maybe in his 2nd or beginning of his 3rd regeneration) being part of Time Lord society before memories of his past companions allow him to recover and forcing the Time Lords to exile him to Earth.
An opponent using a Memory Worm can make things more difficult for the PCs. How do you track someone down when every witness has forgotten what they’ve seen. It could also be used to sabotage the PCs relationships, so that their friends and allies no longer know who they are.
The PCs themselves might be affected by a Memory Worm. They might arrive in a time zone to find out that they’ve arrived after an adventure they don’t remember. Who made them forget and why?
They could even assume that the adventure hasn’t happened to them yet and travel back, only to find that their younger selves are already there. If they were to meet each other it could be the PCs that uses a Memory Worm to remove the encounter from their younger selves memory.
A Memory Worm is also a good way to cover a villains escape. The PCs might believe they’ve seen him die only to find on their next encounter that he escaped and used a Memory Worm to remove this knowledge from their minds, as he wished them to believe he had died.
An adventure can also explore the morality of using the Memory Worm. If a man is the sum of his memories then removing them is akin to mutilation. You are removing a part of who they are.
If the PCs have been freely using the Memory Worm then this can make them re-evaluate what they’ve been doing. They could also have to persuade Torchwood or UNIT to give up the practice.
Finally what if the Memory Worm is not consuming the memories but absorbing them. This could lead to a Memory Worm that becomes a super-genius, able to access the collected knowledge of its victims.
This can lead the PCs to creating a new opponent, particularly if they have been using a Memory Worm mainly on evil people. They would be facing an opponent with all their villainous genius in one mind, even if it is in the body of writhing worm.