The Hath were first seen in The Doctor's Daughter (Series 4 : Episode 6) as part of two groups who had attempted to colonise the planet Messaline in July 6012 of the New Byzantine calendar. The Hath originally left their planet themselves, later joining up with other species such as Humanity with which they shared technology. Alongside humans, the Hath helped mine the Ore required to create the first colonies. During the colonisation of Messaline, the Hath and humans entered war after the death of the mission commander, creating a power vacuum and both species fought to take the planet as their own. After a seven-day battle and the arrival of the Doctor, the two races found the "Source", a terraforming device that would allow them to live on the surface of the planet. Through the process of progenation, the facts of what had happened over the seven-day war had entered mythology, causing both the Hath and the humans to believe the "Source" was the breath of the creator. Following the murder of Jenny by the Human leader Cobb, the Hath allied themselves once more with the humans to continue the colonisation of Messaline.

A Hath with Martha Jones in The Doctor's Daughter

The Hath were a Humanoid race with orange and purple fish-like skin with small headcrests that ran along the top of their heads. They were slightly taller than most Near-Humans, and breathed nutrient liquid rather than oxygen. When they were in a Human-breathable atmosphere they used a breathing structure attached to their mouths. They also had bony ridges around their face and eyes for protection. The Hath were also known for having both a violent streak but also great loyalty, known to be an emotional species they had fearsome tempers, making them dangerous enemies in battle. They were capable geneticists, capable of recreating themselves to be able to live on land and survive with only a small nutrient tank attached to their mouths. Along with the humans, they also possessed progenation machines, which they used to produce new soldiers during their time on Messaline before peace was brokered. During their battle against the humans, the Hath carried heavy duty blaster rifles that fired a barrage of bullets, commonly accompanied by a long plume of flames which was emitted from the barrel of the gun. They also used the same holographic mapping system as the humans, and when the Doctor revealed the suppressed layer of information, it showed up on the Hath's map as well.

A Hath in The Doctor's Daughter

Although the Hath were at war with the humans on Messaline, they were not unfriendly to the Doctor's companion, Martha Jones. Following an explosion, one of the Hath became injured and Martha set out to try and help it, curing its injuries well enough for the Hath to trust her and not try to kill her. When Martha found an above ground route to the "Source", a member of the Hath went with her for her protection. Martha became bogged in a large muddy part of the trek and began to sink, but the Hath member with her saved her, at the cost of its own life. The Hath were also seen in the Zaggit Zagoo bar when the Tenth Doctor met Captain Jack Harkness shortly before the Doctor's regeneration.