Spock mentions all the events that would happen the week of 1968 that they arrived in. Among the events he mentioned was an important political assassination. As it turned out, there were ultimately two important political assassinations in 1968. Just six days after this episode aired on March 29, 1968, Reverend Martin Luther King Jr. was shot and killed in Memphis, Tennessee on April 4, 1968, and two months later, on June 6, 1968, Senator Robert F. Kennedy was killed in Los Angeles, California on the night he won the California Democratic Presidential Primary.
The original draft script didn't even feature the original Star Trek (1966) crew; Gary Seven was an Earthman from the future who had been sent back in time to combat the Omegans, an evil alien people who'd mastered time travel. When ASSIGNMENT EARTH didn't sell as a separate television series, the concept was rewritten into the STAR TREK format
The aliens that Gary-7 represents call themselves "The Aegis". Another word for "shield". Gary's tool/weapon is known as a "servo". The Marvel Comics, fictional agency S.H.I.E.L.D. (Strategic Hazard Intervention, Espionage & Logistics Directorate) was depicted as S.E.R.V.O., which sounds like "brain" (cerveau) in French.
The trivia items below may give away important plot points.
The script called for Isis, when in cat form, to make various cat sounds on cue (meows, purrs, growls, etc.) Since finding appropriate real cat sounds for the soundtrack proved problematic, when the director discovered that Barbara Babcock, who was hired to do the voice of the Beta 5 computer, and for a cameo as Isis in human form anyway, could vocalize convincing cat sounds, Barbara Babcock was called upon to vocalize Isis' cat sounds as well.
Assignment: Earth" was written to introduce a hoped-for spin-off series that never materialized. It would have featured Robert Lansing as Gary Seven, Barbara Babcock as Isis, and Teri Garr as Roberta Lincoln. In the new series, the intrepid three would have worked to make sure humanity achieved the destiny glimpsed via the Star Trek (1966) characters and Seven's mysterious extraterrestrial information. In the episode, Barbara Babcock supplied the cat voice of Isis, but Victoria Vetri, Playboy Playmate of 1968, played the human form of Isis. It is not known whether Vetri would have been available for the spin-off.
Captain Kirk's closing comment - "I'm sure they'll have many interesting adventures together" - refers to the ASSIGNMENT: EARTH series that never materialized. In it, Gary Seven and Roberta Lincoln would have been up against aliens called the Omegans who had mastered time travel, and they would work to make sure mankind achieved the glorious destiny that Gary Seven's alien masters intended it should. To this end, Gary Seven's office computer would have had the ability to turn back time and cancel out any damage that had been done by the Omegans in their attempts to alter Earth's history.