The Hath are a sapient species of humanoid ichthyoids with purplish skin and gill-sets, which must wear a breathing apparatus containing a green liquid mixture in order to survive on land.
A spacefaring species, they were close allies of the Humans circa the sixtieth century (of the New Byzantine Calendar), and the two races engaged in mutual colonization of the planet Messaline. Unfortunately, conflict emerged between the two species when the mission commander in-charge died. With no agreement over who should take charge, the 'power-vacuum' left by the commander's death resulted in a violent war which, although fought over a very short period of time, took the lives of several generations of instantly-created soldiers from both factions.
Eventually, peace was made as - with the help of the 10th Doctor, his daughter Jenny and his companions Martha Jones and Donna Noble - they and the humans found the location of the artifact known as the Source. Upon observing that it was not a weapon as previously believed, but rather a terraforming device, it was used to start colonizing the planet in harmony - with the Source being used to rejuvenate the planet surface.
- The fact that the audience cannot understand the Hath language could imply that it isn't translatable by the TARDIS telepathic circuits.
- In "The Eleventh Hour", when the Atraxi scan the Earth history to search for alien invasions, images of Hath are shown, indicating that they invaded Earth at some point, although that has never been seen or referenced in the series. Other creatures shown in that scene include: Cybermen, Daleks, Pyroviles, Racnoss, Ood, Sontarans, Sea Devils, Sycorax, Reapers and the Vashta Nerada. Many of the clips shown are from episodes not actually set on Earth, but apart from the Hath and the Ood all the other races are known to have been indeed present on the planet before that date (although the Sea Devils, being native to Earth, hardly count - from the Atraxi's point of view the Human/Sea Devil conflicts should appear more like a civil war than an invasion).