WOTAN or Will-OPerating-Through-ANalogue is a supercomputer that appears in the Doctor Who story The War Machines. After a long absence, WOTAN and the War Machines return when they come across the Surgeon in the story Thunderbolt.
The War Machines:
WOTAN is a highly advanced supercomputer and artificial intelligence that eventually beings to think for itself and decides that the world no longer needs mankind, so it takes over key figures and orders the construction of War Machines to take over the world. WOTAN hypnotizes people to make them do what it wants and can even take control of them over the telephone.
In it's return, WOTAN is a lot more modernized and a lot more independent for example: It is equipped with a gas gun which it uses to kill people and revive their corpses and in it's original appearance, while it spoke with a hissing voice, it speaks with a more electronic voice in it's return.
Also, the effect it has on people it has hypnotized depends on the situation. If they have been hypnotized while still alive, then their minds are freed but if WOTAN killed them and reanimated their corpse then they die again, WOTAN also operates more undercover in Thunderbolt and slowly hypnotizes people. It is not until it hypnotizes the Surgeon until it is revealed how many people WOTAN has hypnotized.
It is also capable of taking control of other electronics from a long way away such as telephones, radios and tanks but this is limited only to equipment in Romania. And it's ability to take control of telephones is how it is able to both kill and hypnotize.
While it never appears physically, there is a monologue at the beginning of WOTAN's literature appearance in the story Thunderbolt which details how it was built in 1966 and was housed in the BT Tower or what was once the Post Office Tower and how it hypnotized mankind to build War Machines to destroy humanity and take over the world.
Unfortunately for WOTAN, it was destroyed by the First Doctor which is actually detailed in the story. Now, fifty years later in the present day, WOTAN tries again where this time it is housed in the Palace of the National Military Circle in Bucharest, the capital city of Romania.
Much like the Great Intelligence, WOTAN operates in the shadows and first comes to prominence in the story when it transmits a signal to War Machine 7 ordering it to kill a warehouse worker and lay dormant until further orders.
WOTAN's next orders to War Machine 7 are to find the Surgeon and Amanda and bring the former to it. Once War Machine 7 leaves, WOTAN transmits a signal through the warehouse and activates more War Machines because it has been forced to activate its forces early; later in the story, it eventually activates all War Machines in Romania to kill all humans and transmits a hypnotic signal through the country to paralyze it, the signal also affects the Surgeon.
Under the influence of the signal, WOTAN uses the Surgeon to communicate its plans to Amanda Mycroft and the Romanian sisterhood: The missiles it has captured will be turned into missiles which in turn will be launched into space where they will transmit a signal all over the world that will activate all the War Machines in the world before it finally recalls the Surgeon where it shows him all of the people it had hypnotized during the course of the story.
As the Romanian sisterhood advance on its plans, WOTAN orders the missiles to be launched into space where the missiles turn into satellites and start transmitting the activation signal all over the world but its plans are to be cut short because the Sisterhood reprogrammed War Machine 9 and it is now on its way to destroy the computer.
WOTAN unveils a gun to fight War Machine 9 off with, but to no avail because the Machine sacrifices itself to destroy its former master. Once WOTAN is destroyed, all the War Machines are deactivated and most of WOTAN's slaves return to normal or die because WOTAN killed many and brought them back to life.
WOTAN is one of two villains from the Surgeon stories to be mentioned in Return to Buckinghamshire, the fourth story of The Mammal Wars owing to the return of the Romanian sisterhood in Defenders of Earth and the students from The Yeti Factor in The Mammal Wars, as shown here:
"Britain's Angels, as they called themselves, were a British gang consisting of twelve young English women with ambitions to potentially join the British branch of the Western Vixens.
They were just like the Romanian sisterhood in the East, not because of their ambitions to become Vixens but because you may recall meeting them in earlier stories; for the Sisterhood, you may recall meeting six of them when their country was under attack from supercomputer WOTAN, but that's a different period and a different story.