In the two-part episode The Menagerie, a reworking of the original, unaired pilot The Cage, we learn of an earlier visit of the Enterprise to the planet, during which the Talosians capture Captain Christopher Pike for display in their menagerie and assistance in their breeding program. The Talosians' telepathic abilities are such that they can cause those subject to them to perceive things, persons, and phenomena which do not actually exist, or are unlike or even opposite to their appearance to the observer. Due to the potentially disastrous nature of this threat, General Order #7 made any attempt by Starfleet personnel to visit Talos IV for any reason a capital offense. This was the only such offense remaining under the laws of the United Federation of Planets at the time of the original Star Trek series, but in The Next Generation episode "Justice" Captain Picard reveals that the Federation no longer carries out capital punishment, implying that the punishment for General Order #7 had been changed.
The Talosians' storyline is continued in the Telepathy War story arc in Marvel Comics' Star Trek comic book run in the late 1990s. The story involves a virus created by the Dominion which infects all telepathic species. The Talosians became infected as well, before the crews of the Enterprise-E, Deep Space Nine, and the Starfleet Academy cadets (the cast members of three of Marvel's Star Trek comic series at the time) found a cure. The story culminates with the Talosians taking a large group of Jem'Hadar to be slaves. It is implied that the entire conflict was engineered by the Talosians to this end; they still needed a race to do work for them, as they did in The Cage.