While mining for ore, Will and Smith witness the arrival of a strange beacon from outer space. A voice comes over the beacon, that of a space enforcer, who is sending out a warning to a criminal space swift by the name of Zeno. Smith and Will are wary and baffled, wondering if this Zeno has indeed escaped to their little planet.
As it turns out, Zeno has landed, and by a frightening coincidence, he looks exactly like Dr. Smith. The space enforcer lands, but as Will and Smith go to greet him, Zeno grabs Smith and forces him to trade clothes. Zeno plans to make Smith pretend to be him so Smith will be arrested and hanged in his place. Smith does not like that plan, but Zeno forces his cooperation by keeping Smith in his gunsights.
Smith, disguised as Zeno, goes to the space enforcer and surrenders, claiming he is an outlaw and confessing to all the crimes. The space enforcer, thinking Smith really is Zeno, arrests him and takes him away in his spaceship along with Will as a witness to Smith's confession.
Aboard the enforcer’s ship, Smith tries to explain he is not Zeno at all and only pretended to be because Zeno threatened to shoot him, but the space enforcer does not believe him. Smith tries to get Will to vouch for his identity, but Will is confused and does not know what to think.
Meanwhile, Don and John go looking for Will and Smith and end up finding Zeno, who is pretending he actually is Dr. Smith. The Robot suspects something odd is going on right away, but Zeno threatens to destroy him, so the Robot keeps quiet.
Back on the enforcer’s ship, Smith panics when he learns he is going to be boiled in oil, thrown in a pit of space vipers and then hanged, so he tries to grab the controls and accidently causes the ship to crash. Will and Smith escape the wreckage and look around Zeno’s home planet. Everyone who sees them thinks Smith is Zeno. They are very frightened and treat him very nicely. Will still does not know what’s what, not knowing who Smith really is from one minute to the next.
Smith has quite a lot of fun pretending to be Zeno until a real space swift shows up and wants to duel with him. Then Smith gets cowardly again and he and Will steal a giraffe and a tiger and ride away with a mob chasing after them.
Back on the Jupiter 2, Zeno, still posing as Smith, smokes, flirts with Judy and starts a fight with Major West. Despite this uncharacteristic behavior, none of the Robinsons suspect he is not the real Smith, attributing the change to a bump on the head.
Will and Smith steal the space enforcer’s ship and fly back home. Now confronted with two Smiths, the Robinsons are not sure which one is the doctor and which one is the space swift. The enforcer arrives and decides to sort things out by arresting and torturing them both, but Zeno escapes and hops into a ship and flies away. The space enforcer chases after him, and all is well once more, with Dr. Smith very happy to be himself again.
- This is the second time Smith has had a duplicate, the first being 'Daddy Zack' in "His Majesty Smith".
- This was one of Jonathan Harris' two favorite episodes, the other being "The Cave of the Wizards".
- Not counting the unaired pilot ("No Place to Hide" at 52:10), this is the longest episode of the series, clocking in at 51:51. The shortest episode is "The Wreck of the Robot", at 49:21.
- One of the "WANTED" posters in Space Enforcer Claudius' spaceship is for a "Buck Duane", who had committed crimes on Earth! But Claudius says that Earth is outside of his jurisdiction.
- Michael Fessier also wrote the Rita Hayworth/Fred Astaire musicals "You Were Never Lovelier" and "You'll Never Get Rich."
- In its second season LIS adopted much of the style of its campy ABC competition "Batman." This can be seen very clearly in this episode with the abstract representation of the town, the oversized stuffed animals used for transport, and "funny" signs placed on various items (e. g. "Space Cooler."). It's also evident in the space enforcer's spaceship, which doesn't even have any walls and isn't airtight at all.
- The Robot deduces that Zeno is not really Dr. Smith, because Zeno is holding a gun, and according to the Robot, Smith is afraid of guns. However, in the first several episodes of the series, we saw Smith carry a gun often.
- Even when threatened, shouldn’t the Robot have told the Robinsons who Zeno really was? After all, his main function is to protect them, and having a murderer among them can’t be good for their safety.
- In order to prove to Will who he really is, couldn’t Smith have told Will something Zeno couldn’t have possibly guessed or overheard? Something ONLY the real Smith could know?
- The Space Enforcer can’t tell who is Smith and who is Zeno. He decides to sort the truth out by torturing them or by making them duel. Silly Space Enforcer! What about fingerprints??
- The personalities of Dr. Smith and Zeno were so different it should've been obvious to the Robinsons and Will which man was which.
- How did wimpy old Doctor Smith drink that terrible drink without getting sick?
- And the most baffling oddity of all; the one true consistency about Doctor Smith throughout the series is that he wants to get back to Earth. After riding the animals to escape the mob, Smith and Will get back into the Enforcer’s ship. Earlier the Enforcer clearly stated his ship has an unlimited supply of fuel and star charts of everywhere in the universe. With such a ship in his control, where does Smith take it? Back to the dismal little planet where the Robinsons are stranded! Why didn’t Smith drive himself and Will back to Earth?? Or at least try to.
- Allen Melvin guest stars as Space Enforcer Claudius. He played many minor roles on TV including The Andy Griffith Show, Gomer Pyle and The Brady Bunch, but most importantly is remembered as the voice of Drooper in The Banana Splits.
- Zeno's use of the nickname "pussycat" with everyone is really too much, especially when he's saying it to Dr. Smith in the beginning. I suspect Jonathan Harris had a lot of fun with that. Likewise, the reference to "cauterizing" Dr. Smith's "giblets" is just too much!
Zeno: "You do like I told ya.... Or would you rather I cauterized your giblets right here and now?!" Smith: "Well...any alternative is better than none."
Claudius: "Give me your keys, I'm going after them." Pleides Pete: "Watch it, it slips a little in 2nd stage."
Penny: "Gosh, two Dr Smiths!" Don: "That's two too many."