Seven and Roberta meet and come into conflict with Isis and Harth, setting up the series' premise. After their initial adventure together involving going back in time to reset a mishap and Roberta transporting instantly around to different locations, Seven tells Roberta he needs an assistant.

Seven's cover in the 1960s is The -7- Agency, a private investigations firm. We meet Roberta as she enters the office looking for Mister Seven. The gadgets from the final episode are here, including the servo, and a pair of working x-ray glasses. She sits down at the typewriter to leave him a note. Roberta had nearly been killed by a falling chunk of a building, and had been pushed out of the way by a woman who instead died. The woman looked very much like her, and Roberta found Seven's address on her body.

Gary Seven is a man sent back in time from the 24th century, the only Earth man to ever survive the transit. His goal is to defeat the Omegans, a race of shape-changing aliens who have sent agents back in time to change Earth's history so they can defeat Earth in the future. Harth and Isis would be the primary Omegan antagonists. Roberta Hornblower is described as she appeared in the final episode, but as a 20 year old.

Gene Roddenberry developed the first version of Æ as he worked on Star Trek's first season, and pitched it to Desilu in a 47-page script.

Some of the connecting-the-dots promotion of the series' ideas to already known commercial quantities is a bit funny to read now. Having done enough creative briefs and seeing the tell-tale signs in this proposal, I get the feeling studio execs have the same thought processes as other businessmen.

One of Roddenberry's strengths and benefits was to go to specialised individuals and organisations (like NASA) and ask them, "What if?" By going outside entertainment circles, he gave his work a depth and credibility that became a model for a better-informed process.

Now conceived of as a Star Trek spin-off pilot, the new Æ had Roddenberry and Wallace selling themselves as individuals respected in the business who were teaming up for the series. They made the clear distinction that while futuristic like Trek, Æ would be set against modern-day 1968.

About 30–50 per cent of the Seven-Lincoln-Isis story is not developed yet. It feels much more like a Trek episode with Seven and Lincoln as guest stars, instead of the back-door pilot it became. A lot of re-writing was done over the holidays by Art Wallace to deliver the episode we know.

Roberta London, recruited by Mr. Seven, was beamed up to the Enterprise for interrogation. The frightened Roberta was soothed by Uhura, who reassured her that she was still among Earth people.

After Seven was confined in the Enterprise brig, he revealed his mission to Dr. McCoy, turning the tables on Bones by asking him to think like a doctor, not a mechanic.

Gary Seven's transporter beam came from even farther across the galaxy than it did in the episode.

No black cat! Isis – either human or feline – is nowhere to be seen.

There were some differences from the final episode in this version:

"Assignment: Earth is interesting in a sense," Wallace points out, "because I had gone to Paramount and pitched a series idea to them. They had said that Gene Roddenberry had come up with a very similar idea. So I saw Gene and we decided to pool the idea, which was about a man from tomorrow who takes care of the present on Earth. That was intended to be the pilot, although it was never made into a series. It was a good pilot and it's a shame, because I think if they had done it as a series with just Gary Seven, it would have been a very successful show."

In the middle of Star Trek's second season, Roddenberry and writer Art Wallace reworked the Æ premise:

Notable changes from the final-draft script to the produced episode include:

The supplemental Captain's log which immediately follows Seven's capture where Kirk describes "A man in a 20th-century business suit. What is he? Not even Spock's…etc." was not in this script.

In the briefing room, a line by Spock is cut:

Spock: Medi-scanners indicate it is a cat, Captain. Female… as we've seen, remarkably intelligent…

McCoy was to enter the briefing room scene earlier, with Kirk showing impatience with him to report.

Just before the Beta 5 says, "In response to nuclear warhead…" an exchange between Seven and the Beta 5 is cut:

Seven: Computer, how much longer?

Beta 5: Useless questions will only prolong search.

Seven: Are you a one-relay machine? Clear a circuit; describe present mission of agents 201 and 347.

Immediately following Seven saying, "That's the same kind of nonsense that almost destroyed planet Omicron IV," a line has been cut:

Seven: Balance of power won't work. The other side will launch still more, they'll end up with the sky full of H-bombs waiting for just one mistake.

The scene where we first see Roberta Lincoln was scripted to include Kirk and Spock in the background, following her. In the episode we see Roberta make a comedic entrance, and Kirk and Spock travel the same sidewalk a few minutes later.

When Seven poses as a CIA agent to Roberta, some of the dialogue was softened to make it a more friendly exchange. Originally, it was to be more combative, as it was in the first part of this scene.

After Seven transports out from his vault, the scene with Kirk, Spock and Roberta has been restructured. The three were scripted to come into Seven's private office together, they weren't aware of the vault transporter, and it was Spock who found the map of McKinley Base. In the episode, Kirk rushes into the office alone, sees the vault close before he can reach it, and brings the map back out to Spock and Roberta in the outer office.

During the scene with Sergeant Lipton phoning in the security check on Seven, Isis was scripted to be following Seven. Knowing cats, this was most likely impossible to accomplish on set, and so Seven carried Isis and the unscripted line for Seven to put down the cat was necessary to have her under foot to finish the scene as written.

Seven and Isis on the gantry arm is unscripted, though what they're doing is detailed. As written, Seven and Isis walk out of the elevator in one scene, and in the next Seven is removing the panel. Perhaps Wallace did not describe the exact environment because he knew that it would depend on matching the stock footage supplied by NASA with the sets that Desilu would build in response, and that happened after the scripting process was completed.

The cigar box Roberta uses to konk Seven in the back of the head was originally scripted to be a heavy art object. Given Teri Garr whacked Robert Lansing with the small padded box hard enough for the actor to see stars, it's probably just as well.

The call from Scotty to Kirk about all powers being on alert was scripted for Spock earlier in the scene.

Roberta was to lower the servo on her own, rather than having Seven intervene. As shot, the scene works better, building trust between Seven and Kirk.

Roberta's plea to Kirk, "He's telling the truth." was to have another piece:

Roberta: A woman feels things about a man.

Spock: A point against him, Captain. They are usually 100 per cent wrong.

Probably a good idea to have excised all that.

Kirk says, "Spock, if you can't handle it I'm going to have to trust him." As scripted:

Kirk (agony): Spock, it's all mankind at stake. No man should have to make this decision.

During the wrap-up, a whole piece of the scene was removed:

Kirk (glancing at Roberta): One other thing is needed to maintain history as it is supposed to go, Mr. Seven. A permanent secretary. (indicates) Our historical records indicate that one Roberta Lincoln resided at this address many years.

Roberta: 'Resided'? Now wait just one minute, friend…

Seven: Living here will be no threat to your 20th century moral code, Miss Lincoln…

Seven: It's a separate adjoining apartment which was leased for Agent 201… You'd find it quite luxurious…

Much of this happens while Roberta is looking at the human Isis, and as such, it probably didn't work because everyone else's attention was on Roberta and they would have seen Isis too.

After the "Simply my cat, Miss Lincoln" gag, Roberta's living arrangement dialogue continues:

Seven: Can you use the apartment? It would be convenient for the new agents to have a secretary nearby.

Seven (to Kirk): I expect to be replaced shortly. Your record tapes showed other names listed at this address. (waits, then frowning) They did, didn't they, Captain?

Kirk: I afraid we can't tell you everything we've learned, Mr. Seven. (glancing at Roberta, back at Seven) It might change history if you knew too much.

The line Spock says about "interesting experiences in store for Seven and Lincoln" is absent from the script, and was most likely used to replace the longer explanation for a quicker and cleaner wrap up, and perhaps to leave things more open ended for how Æ might eventually be produced.

"Assignment: Earth" aired as the last episode of Star Trek's second season. It failed to generate interest, and the series never materialised.