Tagra IV was the inhabited fourth planet in its star system. The system was one of six located within the Argolis Cluster. This planet was the homeworld of the Tagrans, a humanoid species.

By 2369, the Tagrans' industrial activities had polluted the atmosphere of Tagra IV to a critical point, despite their installation of a thousand barystatic filters in an effort to clean the air. These filters had caused a severe ionization effect in the planet's atmosphere, preventing the function of transporters.

On stardate 46192.3, the USS Enterprise-D mounted a relief mission to assist Tagra IV, one of the largest such missions they had ever mounted. This included the installation of a new field modulator in the reactor that powered the barystatic filters on the southern continent, which had been experiencing difficulties which could have required it to be shut down.

However, the damage to the reactor was worse than the Tagrans originally described, and it continued to destabilize even after the field modulator was installed. Lieutenant Commander Geordi La Forge attempted to avert an overload with a neutrino infusion to smother the reaction, without success as the heat had fused the reactor injectors. A meltdown seemed imminent, threatening thousands of people in the area.

At that time, Amanda Rogers aboard the Enterprise exercised her abilities as a Q to stabilize the reactor, expressing that she could not allow so many people to die in spite of her previous pledge not use her powers. She then restored the ecosystem of Tagra IV to its natural state, dropping the atmospheric pollutants to less than one part per trillion. Realizing that she could no longer live as a Human, she then returned with Q to the Q Continuum. (TNG: "True Q")

According to Star Trek: Star Charts (p. 47) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the Tagra system was located in the Alpha Quadrant. The system's primary was a G-class star.
According to StarTrek.com, Tagra IV was a M-class planet.
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