In the distant past, the planet Zetar boasted a large humanoid population but, millennia ago, all life on the planet is thought to have been destroyed. This is not, however, entirely true.

As their race faced extinction, the last of the Zetars understood that all things must die, but did not believe that the death of their planet was also the right time for the death of their race.  They could not allow themselves to have wiped out, and so began to formulate plans to ensure their survival.

At the peak of the Zetars` preparations to leave their doomed planet, a final, sudden disaster hit. This struck them down, but it could not defeat them. All corporeal life ended, but 100 Zetars survived, transformed into a strange noncorporeal yet sentient energy. Their physical bodies were lost, but their life force continued.

The state in which the Zetars survived the death of their planet was very different from the one in which they had existed until then. Doomed to wander through space in their new, light-like state, they spend the next millennia searching for a compatible physical body through which they can see, speak, and hear, and through which they can live out their lives.  Their only concern is for their own continued existence.

In their noncorporeal form, a number of Zetars form what appear to be, at first glance, a coloured cloud of lights floating in space. The cloud has two thick bands of red and green light around its exterior; these surround numerous flickering, bright white smaller lights. The cloud sends out high intensity energy readings, and is similar to a storm, for which it can be mistaken.

The cloud, however, differs from a storm in several important ways. Firstly, it is capable of travelling at speed in excess of warp factor 2.3, whereas no natural phenomena can travel faster than the speed of light, it exists in the vacuum of space, where the conditions to support such a natural phenomenon do not exist. Most importantly, the cloud registers as its component life forms on sensors; each life sign within the cloud can be described as powerfully alive and vital. The clouds acts in a way impossible for a natural phenomenon. It can match any evasive manoeuvre a starship makes to avoid it, and can pass through shields with ease.

While not openly aggressive, the Zetars have little regard for life other than their own. Having lost their own physical state, their aim is to find other physical bodies they can possess - humanoids with minds which are compatible with their own. They go about this by sweeping over starships or planet-based facilities in their path, locking into the minds they encounter. They need to find one which is easily adaptable; such minds are often those belonging to people who are susceptible to learning and assimilating new experiences easily. When they have chosen an individual, and image of the lights can be seen reflected in the affected person`s pupils. Physically, the possessed victim is uncharged; the only indication that there is anything wrong is that his or her brainwave pattern will have changed to match that of the group mind of the cloud.

When in possession of a physical body, the Zetars can then speak through the victim and thus communicate with others, at first, the sounds coming from the victim`s mouth sound like gibberish, a low, gargling noise. Then, his is her face begins to change colour, glowing with a series of bright lights which reflect the colour observed in the cloud. At this stage, there is one of two outcomes; either the victim can accept the possession, or try to fight it. Their results in death, caused by severe brain haemorrhage brought on by the distortion of all neural systems and the dissolution of the autonomic nervous system. The exact area of the brain affected varies from victim to victim, but all who try to fight the possession die.

If the victim offers less resistance, the Zetars` efforts to communicate become comprehensible, and their voices are heard through the victim`s mouth.

When not possessing a physical body, the Zetars do not respond to hails from starships, even if the universal translator is used. It is unknown if this is by choice or because doing so is impossible.

Once inside a body, the Zetars have no intention of giving control back to the rightful owner. They understand that this effectively signals the death of the possessed victim, but consider their own right to life to be more important. As such, they present an unacceptable danger to the humanoid life forms of our Galaxy.

The last known surviving Zetars are destroyed by the crew of the USS Enterprise in 2269, following a series of ruthless attacks on Federation facilities and personnel.