From the sci-fi series Stargate SG-1. The Tok'ra are rebel Goa'ulds who oppose the system lords. Their name literally means “against Ra,” as they were initially named for their resistance to Supreme System Lord Ra, who was killed by the Tauri (aka Earthlings) in the Stargate movie.
Like other Goa'ulds, the Tok'ra are symbiotes, or parasitic creatures that live within a humanoid host. Normally, when a Goa'uld implants itself within a host, the larva takes full control of all the host's actions and thoughts and little (if anything) of the host's original personality remains. The Tok'ra, however, do not believe in taking over the body of an unwilling host, preferring instead to find willing individuals who wish to blend their personalities with the personality of the symbiote.
The SG-1 team's first experience with a Tok'ra was in episode #202, In the Line of Duty, when Carter is taken over by a Tok'ra named Jolinar of Malkshur. When Carter is later attacked by a Goa'uld assassin, Jolinar allows herself to die rather than risk Carter's life, distinguishing the Tok'ra from common Goa'ulds, who generally consider the lives of their humanoid hosts to be fairly inconsequential.
In episodes #211-2, Tok'ra Part 1 and Tok'ra Part 2, the SG-1 team worked to convince the Tok'ra to ally themselves with the Tauri in their war with the Goa'uld system lords. Feeling that humans had little to offer them in return for the alliance, the Tok'ra were fairly cold to the idea. To aid in the diplomatic effort (and to avail himself of the symbiote's regenerative properties), Samantha Carter's ailing father, General Jacob Carter, accepted a blending with a Tok'ra larva named Selmak. Shortly thereafter, the Tok'ra agreed to join Earth in its fight with the system lords and Selmak/Jacob became a recurring character on the series.