When a natural-gas complex began drilling in the North Sea, they ended up extracting more than just gas. A bit of seaweed was drawn up by the drilling rig, and blocked the bore hole. When an engineer went to clear out the blockage, he came into contact with the seaweed...and thus began the invasion of the Earth.
The Weed is some kind of large seaweed creature that lives in the North Sea. It is actually a living entity, as much as any human. Mariners reported sightings of it as early as the middle of the eighteenth century, but it was believed to be merely a legend. It is apparently able to separate itself into smaller pieces of seaweed without harming itself, and these smaller pieces enable it to spread itself much further. They can also re-form into a larger entity. The Weed feeds on the natural gas beneath the sea, and when exposed to the gas, it grows larger in size.
The Weed travels in masses of seafoam, which is found abundantly in its presence. In fact, the Weed produces so much foam that the places where it dwells are swamped with it, even if the Weed is physically elsewhere. It may lie dormant in the foam until someone approaches, at which point the foam bubbles up toward them and the Weed within begins to thrash about. When the Weed wishes to move someplace, it is carried along by the foam, which makes a kind of sucking, bubbling sound as it moves. The foam can carry the Weed through ventilator shafts and out the grilles into a room, and then recede without a trace.
The presence of the Weed is signalled by a deep, throbbing heartbeat sound, which echoes through the pipes in which it travels. The sound grows louder as the Weed approaches, and is very prominent once the Weed has actually emerged into a room. It also has a defensive weapon - a kind of toxic, non-natural gas that hisses from the Weed. A mild exposure causes sneezing; prolonged and intensive exposure causes choking and, presumably, unconsciousness or death. The Weed is also very strong, able to drag people by the leg and push against solid objects.
If a human's skin comes into contact with the Weed, the touch stings them, although it leaves no mark behind. Once stung, the victim develops a telepathic connection with the Weed, enabling them to hear the creature's heartbeat and also enabling the creature to mentally control them. It knows everything that the people under its control know, and its victims behave as if they are in a trance, barely retaining the will to fight. But the Weed is able to speak through their mouths, allowing them to pass more or less unnoticed, aside from the fact that they refer to themselves in the plural "we". They also have slight difficulty moving, possibly because the Weed's control of their motor functions is not as strong.
Technically, the Weed is a parasite, attaching itself to other living things. Apparently it gets its intelligence from human brains. After a person has been under its control for some time, Weed begins to grow from their skin - their hands, in particular - and they can emit the toxic gas from their mouths, accompanied by the hissing sound. This may be the initial stage of transformation from human into a seaweed creature, but this is never confirmed. The Weed also enables the humans to travel underwater without being harmed. But touching a person infected by the Weed does not pass on the infection; apparently this can only be accomplished by coming into contact with the seaweed creature itself.
The Weed is generally very powerful. Attempts to destroy it with explosives would simply allow it to spread further. But pure oxygen is toxic to the Weed and to the humans under its control. Its other weakness is sound vibrations, specifically loud noise. Given the general level of noise within the gas rigs, it seems likely that loud human voices are the key. Humans controlled by the Weed only speak in whispers. But the noise that is apparently the most dangerous to it is the specific sound pattern of Victoria Waterfield.
Once the Weed had established control over the engineers who tried to extract it from the drill, it began to take over top-priority people first. Eventually it took over two, then three, of the impeller rigs. The crew of the control rig noticed that the pipes were blocked and the gas pressure had fallen, but they attributed it to mechanical failure, despite the heartbeat sound in the pipes. The Weed eventually reached the control rig, and its operatives emptied all the oxygen containers in the oxygen storage room. Slowly, one by one, it began to take over the members of the base. The Doctor realized that it intended to take over all of the rigs, form a giant colony, and eventually take over the British Isles and then conquer the world. In order to destroy the Weed, the Doctor realized they would have to destroy its nerve center, so he set about figuring out how they could generate enough sound to penetrate the control rig and kill the nerve center. His solution was to make a looped tape recording of Victoria's scream, the one sound in particular which seemed to be fatal to the Weed. With this done, he amplified it as a sonic laser sound wave which could be passed through the pipe to destroy the nerve center. To combat the Weed that was already on the control rig and thus unreachable from the pipes, the Doctor rigged speakers to point at it. In the end, Victoria's screams killed the Weed, making it disappear from all the rigs and restoring everyone who had been under its thrall.
