Since we’re only a few minutes into the episode there is very little information to go on and so it is all wild speculation. We later learn that the cubes are both data gathering devices and a means to execute the human race.
This means that the other possibilities can be explored in your own games. Let us look at these alternative purposes of the cubes and how they might be used in your campaign.
“What If They Are Bombs?”
If the cubes were explosive then they would work extremely well. We see that the cubes appear through miniature wormholes, allowing one to appear in the Pond’s bedroom. Nowhere would be safe or protected from the potential blast.
Worse still people took the cubes into their homes. Doubtlessly some people probably carried a cube with them. This makes people even more vulnerable should the cube turn out to be a bomb.
This being science fiction the potential explosion could have been small (causing minor structural damage or physical injury) to unimaginable (atomising the Earth). On top of this the combined blast has the potential to do even worse damage.
The explosion might also destroy the environment or introduce a toxic element. Even if the blast was relatively small if it was radioactive it could render areas completely uninhabitable.
If PCs did learn that the cubes were bombs could they co-ordinate with UNIT and world governments to get rid of them in time. Even if they could get them away from people where would put the massive number of cubes before they went off? Could they find a way to defuse them?
This option maintains much of the original intent of this episode. They are still designed to kill, just in a different way and with large scale property damage.
“Or Transport Capsules?”
If the cubes were actually space crafts then this would have turned ‘The Power of Three’ into a more typical alien invasion story. Given the dimensions of the cube the aliens could be very small, with the invaders making up what they lack in height with sheer numbers.
This being Doctor Who the cubes could actually be much bigger on the inside, allowing for larger aliens. This could be the aliens clever technique of getting inside homes and shelters without being noticed.
The alien could be peaceful, simply spending the year studying the humans that take their cubes. They might judge their subjects, deciding whether humanity is ready for their presence and the knowledge they can offer.
An adventure could be built around how the worlds reacts to the cubes, which ultimately effects the aliens judgement. If the world nations squabble over the cubes, fighting over their control the aliens might deem humanity unworthy and leave (or try to wipe us out) but if they co-operate the aliens might extend the hand of friendship. The PCs could be vital in determining what they do.
“Or Deadly Hard Drives?”
The idea that the cubes contain information has a lot of potential. Each cube might contain some incredible piece of knowledge that could radically alter human culture. People might uncover incredible secrets, powers or technology.
This could result in chaos as people abuse the knowledge they’ve been blessed with. Governments and organisations might fight over the cubes or try to regulate the information they provide.
Everyone would wonder what information everyone else had gained from their cube and where they can get more. Would they trade for them or try to take them by force? Would people work together to transform society or conquer it?
The PCs could be caught up in the hunt for more cubes or they could try to prevent things from getting out of hand. It would be their job to stop newly created mad scientists and things that man was not meant to know.
This could be the intent of the cubes creators or an unintended result. The cubes might be a gift or devices sent out to gather information only to find themselves on the wrong planet.
“Or Alien Eggs?”
A similar idea to the concept of the cubes as transport capsules. Given their geometric shape it would be unlikely that the cubes are produced by an alien species. Still, they could be artificial sculpted into that form or it could be result of travelling through wormholes.
If the resulting species were non-hostile this could be a good way to introduce an alien species to Earth. This infant race might be unaware of their origin and humanity would have to decide what it was going to do with their rapidly growing and hungry alien species living alongside them.
Over the years this orphan race would wonder who they were, where they came from and what their identity should be. Much of how they developed would be determined by humanity.
Would people accept them or call them monsters? Would they be turned into slaves, worshipped or exterminated? All interesting areas that could be explored in an adventure based around themes of immigration, race and tolerance.
The aliens might be parasites, allowing potential hosts to take them in. Once their prey is asleep the aliens hatch and strike. This could happen slowly or all at once. Do they take over their host completely, alter their behaviour or give them beneficial abilities?
In such an adventure PCs will need to study and respond to the parasites. Can they find a cure and will they become infected themselves?
“Or Messages Needing Decoding?”
In this scenario the cubes are bottles cast into the infinite expanse of space like messages in a bottle. The difference in this case, as opposed to the data drives, if that the origin has something to communicate rather than knowledge to bestow.
They might have been created by an alien race that were unable to travel in space. Instead they send out these messages, letting other species know that they are not alone and hoping that one day they will seek them out.
They could be an intelligence test. If a species can decode the message and follow the contained instructions they are worthy of contact. The PCs could be involved in trying to crack the code.
The message could be a threat. What if the cubes announced an invasion or promised that the writer of the message would be coming for Earth? Is it an empty threat or psychological warfare? Would Earth be able to ready itself for attack?
It could be that the message was never intended for Earth and encrypted to prevent it from falling into enemy hands. The PCs could be responsible for taking the message to its intended recipient or keeping it out of the wrong hands.
“Or Part Of A Bigger Whole?”
The cubes could combine together into different combinations to create something much bigger. They could be component parts of a single alien or weapon of mass destruction. Individual data or messages might reveal even greater information when assembled.
In such a scenario the PCs might be either trying to put the blocks together or keeping them apart to prevent a terrible threat. Such an adventure has built in tension as the players wonder what will happen when all the pieces of the puzzle are put together.