Chancellor J'mpok subsequently declares war on the Hegemony. Kang returns to Qo'noS with evidence showing that Undine infiltrators are controlling the Gorn Hegemony and that they are responsible for the current conflict with the Klingon Empire. Cochrane is relieved of duty, along with two senior officers, and detained at Facility 4028. The shape-shifter reveals that it is an Undine, and that it was sent to kill and subsequently impersonate Ja'rod to infiltrate the Klingon High Command. He kills two of them and manages to incapacitate the third for further interrogation. Ja'rod, the leader of the Klingon House of Duras, is attacked by three shape-shifting aliens during his shore leave on Rha'darus.

The Undine is killed by a Starfleet security team. Raka, a lieutenant of Hassan the Undying, is revealed to be an Undine infiltrator after being exposed to catalyzed decalithium.It is suggested that the reversion was caused by an excessive amount of alcoholic Meridor consumed by the Undine. The infiltrator is swiftly killed by the other members of the Gorn team. During a baseball game between the Pike City Pioneers and the S'Yahazah City Talons, a team representing the Gorn Hegemony in diplomatic negotiations with the Klingon Empire, the pitcher for the Talons abruptly reverts to Undine form.He manages to defeat his attacker, but is forced to disintegrate the body. While recovering on Vulcan from wounds sustained during a battle against Nero, Worf is attacked by an Undine.Voyager eventually discovers this, and Captain Janeway is able to open diplomatic channels and negotiate a cease-fire. Having identified the Federation as a greater threat than the Borg, the Undine prepare to infiltrate Starfleet using terraspheres as training ground for their agents.After returning to normal space and defeating thirteen more, the Undine determine that the invasion is not worth the price and withdraw their ships. Equipped with enhanced Borg technology, Voyager enters fludic space and destroys four Undine vessels in a show of force.They form an alliance with the Collective, helping them to develop technology to use against the invaders in return for safe passage through Borg space. The crew discovers that the Undine intend to wipe out all life in the galaxy, and therefore represent a greater threat than the Borg. Voyager - having been stranded in the Delta Quadrant three years earlier - wanders into the crossfire and narrowly escapes destruction at the hands of a Nicor Bioship. The plan backfires, as the Undine are immune to their attacks and begin striking back at the Collective's territory in the Delta Quadrant. The Borg discover the existence of the Undine, and invade fluidic space in an attempt to assimilate the species.Undine ships use Fluidic Antiproton weapons. They are also capable of shape-shifting using a form of genetic manipulation, and can switch between different forms at will. This makes their vessels unusually resilient against Borg attacks. The Undine utilize organic technology comprised of the same material they themselves are made of. They see all other creatures as inferior. They are quite xenophobic, and rule their part in fluidic space with force. The Borg consider them to be 'the apex of biological evolution'.

They stand approximately 3 meters tall and have a tripedal structure (three legs). Their DNA is formed by a triple helix which makes them very resistant to almost all forms of biological attack, including Borg assimilation.

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