The 456

The 456 (pronounced 'four-five-six') was an alien race that negotiated secretly with the British government in 1965, and again in 2009. This designation was given to them by humans based on the radio wavelength they used, and they took on the name as their own in their dealings with humans rather than offering the actual name of their species, if any ever existed.

As they later revealed, they were drug dealers. The 456 used children as "the hit" because the bodily chemicals they create "felt good".

Biology
Very little was known about the 456's biology and anatomy, as humanity only had contact with one member of the species, who remained within a nearly opaque gas, contained within a transparent chamber. The natural design of their body consisted of three "heads", connected to one body via three long necks, possibly a conjoined triplet species. It resembled a large, plucked, three-headed swan. (TV: Children of Earth: Day Four) The 456 thrived in an atmosphere made up of 25% nitrosyl chloride, 22% hydrogen chloride, 20% nitrogen, 12% fluorine, 9% hydrogen cyanide, 6% acetone, and 6% phosgene. (TV: Children of Earth: Day Two)

It was later revealed that the individual may have been under the influence of a recreational drug the entire time, the effects of which were not fully known. The 456 ambassador emitted a viscous green fluid in moments of frustration; this may have been an effect of either the use of recreational drugs or a natural response. (TV: Children of Earth: Day Three) Their blood was red, though this may have been from the children. (TV: Children of Earth: Day Five)

Technology
The 456 had technological systems which could match the systems of the human race from 1965 onwards, such as radio telecommunications. They spoke English fluently and understood it perfectly; this may or may not have been through the use of a translator. They seemed to be far more advanced than the human race, indeed far more advanced than most other species, having the chemical and biological knowledge to provide a range of viruses and anti-viruses to humans. (TV: Children of Earth: Day Four)

An orphan from 1965 connected to the 456 Ambassador. (TV: Children of Earth: Day Four) The 456 used human children as a source of drugs. They would forcibly attach a child to their body with a life support system. This allowed the child's chemicals to stimulate feelings of pleasure in the 456. During the process - according to the Ambassador - the child felt no pain and lived long beyond its natural lifespan. However, it was conscious throughout the experience and had to use a respirator to survive in the toxic atmosphere. (TV: Children of Earth: Day Three, TV: Children of Earth: Day Four, TV: Children of Earth: Day Five)

The 456 arrived on Earth in what was described as a "pillar of fire", allowing them to teleport into a specially made tank, despite it being within a building. UNIT believed that their base of operations was in orbit around Earth, but they were unable to find it. (TV: Children of Earth: Day Three) In 1965 the 456 took the children in a similar way, teleporting them away in a flash of light. (TV: Children of Earth: Day Four)

They could also manipulate the minds of millions of children at will. When they came to Earth in 2009, they forced all children globally on Earth to chant "We are coming". The transmissions became more and more sophisticated with each sending, until the 456 manipulated each country's children into relaying a different message to their governments. Children around the globe communicated in English only, due to the 456's previous dealings with the British government. This connection they could also give off a fatal signal, which they used to kill Clement McDonald. (TV: Children of Earth: Day One, Children of Earth: Day Four) However, if reversed, the frequency was deadly to the 456, making their bodies exhibit extreme pain before violently exploding. (TV: Children of Earth: Day Five)

First contact
The 456 first made contact by radio on the wavelength sometime during or prior to 1965. The 456 had learned English and knew something of Earth culture.

In 1965, Andrew Staines, Ellen Hunt, Michael Sanders, and Jack Harkness executed a deal with the unseen aliens. In exchange for twelve human children, the 456 gave them an anti-virus for a mutated strain of Spanish influenza, which would otherwise, according to the aliens, have killed millions. These projections were agreed to by British analysts. The 456 claimed that the children would not be harmed, and in fact "would live forever" and that the 456 would not return to Earth. (TV: Children of Earth: Day Three, TV: Children of Earth: Day Four)

What governmental bodies the first three people represented remains unclear.

Twelve orphans were taken from the Holly Tree Lodge orphanage under the pretence that they were to go to another orphanage called Harbour Heights. These children were brought to a location in Scotland, where they were ushered into a white light. For unknown reasons, the 456 decided to leave behind one orphan, Clem McDonald, who ran off alone into the night. Torchwood Three later speculated that they weren't interested in him because he was on the brink of puberty. (TV: Children of Earth: Day Four)

Initial contact
In September 2009, the 456 sent a message through every child—ages five to eleven—on the planet: "We are coming." (TV: Children of Earth: Day One Children of Earth: Day Two) They followed with detailed instructions sent secretly to Dekker for a purpose built, gas filled chamber to be constructed for them on Floor 13, the top level of Thames House, MI5's headquarters in London and the most secure building in Britain. Technicians set about constructing the containment facility before the aliens' arrival. (TV: Children of Earth: Day Two)

The 456 arrive on Earth. (TV: Children of Earth: Day Three)

Arrival
On their arrival day, an ambassador transported itself from the sky in a pillar of fire into the chamber, and began negotiations with John Frobisher, whom Prime Minister Brian Green had had appointed informal ambassador. They insisted on addressing all the major powers, but Frobisher was able to convince them to keep quiet about the earlier dealings with the British government in 1965. The 456 asked for a "gift" of 10% of the world's children, in exchange for not destroying the human race. (TV: Children of Earth: Day Three)

The request was denied and the UK government offered sixty-seven hundred children of Failed Asylum Seekers; the 456 rejected this offer and used the children of each country to tell the respective governments the number of children required by the 456.

The arrival of Jack Harkness led to an argument with the 456, who released a poison into the air of Thames House as a demonstration of what the people of Earth would get if they didn't comply. This killed almost everyone in Thames House, including Ianto Jones and (temporarily) Jack Harkness. The 456 gave the world governments one day to consider. (TV: Children of Earth: Day Four)

Resolution of the crisis
The government continued with its operations to round up the lowest performing 10% of the UK's children to give to the 456. Before the 456 could take their prize, Jack Harkness was able to turn back on the 456 the radio wave which had killed Clement. Doing so either killed or otherwise repelled the 456, saving the Earth. It is unknown if any of the other 456s were killed. The individual at Thames House may have exploded, but the contents of the gas chamber were removed in another "great pillar of fire" before it could be investigated. To effect this plan, Harkness reluctantly but knowingly killed his own grandson. (TV: Children of Earth: Day Five)

The 456 would become the namesakes of various resolutions and changes to laws across the globe, including the 456 Regulation in Great Britain and the 456 Amendments to U.S. Code 3184. The 456 Regulation declared the members of Torchwood Three officially deceased, and the 456 Amendments allowed the extradition of the Torchwood team into the United States when captured. (TV: The New World)

Behind the scenes

 * Humans used the name "456" to refer both to the Ambassador and to the species generally.
 * The loud screech made by the 456 was previously used by The Swarm in "Planet of the Dead".