I was watching a few scenes of “Tomorrow is Yesterday,” a classic 1967 episode of “Star Trek,” the other week. My son looked up from his iPod during a fight scene.

“That is so cheesy,” he said, his voice dripping with good-natured scorn. “He didn’t even hit him.”

He was right. The on-screen fight scene between Kirk (William Shatner) and a group of military police officers didn’t rank up there with the most realistic screen pugilism ever. Kirk draws back, throws a right cross … and visibly misses the MP by a mile.

But still.

“Tomorrow is Yesterday” is a great episode of the original series and a great episode of TV science fiction.

Little more needs to be said about what made “Star Trek” as great and enduring as it is. But the exchange with my son made me think about the differences between good TV sci-fi and bad.

So, in a blog entry that will, with any luck, be recurring, a few thoughts on a good sci-fi TV series as well as one that’s not so good.

And yes, I have few doubts that even the series that I choose to pick on here have fans. And I’m a fan of some elements of even those shows, and I’ll cite those elements. But there’s no comparison between the great ones and the not-at-all-great-ones.

This time around: “Star Trek” vs. “Lost in Space.”

(Some of) what makes “Star Trek” great:

1. The show employed some of the greatest writers working in TV and science fiction in the 1960s, and they produced great scripts. Robert Bloch’s “What Are Little Girls Made Of” was an ultra-creepy tale of android love. Theodore Sturgeon’s “Shore Leave” showed that “Star Trek” mixed whimsy and suspense better than anyone. Frederic Brown’s “Arena” was adapted into a gripping episode featuring Kirk one-on-one with a man-sized lizard (hampered only by the makeup and costume limitations of the day).

2. Episodes were so good they were not only memorable for decades to come but provided fodder for sequels and remakes. “The Trouble with Tribbles” spawned a cottage industry in homemade fur balls — as well as enduring love — among fans. “Space Seed” created a memorable character in Ricardo Montalban’s Khan, who inspired the best of the “Star Trek” big-screen roles.

3. Episodes were as formulaic as much of what appeared on TV in the mid-to-late 1960s but transcended most of the competition to prove as lasting as anything ever on TV. Even with today’s mania for remaking old pop culture, only a handful of shows from the time — “Mission: Impossible” comes to mind — are still in the public mind. How’s that big-screen version of “The Virginian” coming?

4. The show was remarkably consistent to its characters. How many shows before, during and after were filled with characters who veered wildly between sensible and nonsensical, bold and mild, jokey and humorless depending on the plot contrivances of the week? Not “Star Trek.”

5. And speaking of characters: “Star Trek” creator Gene Roddenberry and the writers decided they needed a strong triangle of characters to lead the show, so they created man of action Kirk, cerebral Spock and emotional McCoy. Viewers could enjoy the interpersonal dynamic but the triangle also served the plot, with McCoy and Spock acting as antagonists, virtual angel and devil on Kirk’s shoulders, the voices of reason and emotional appeal.

6. Here’s a bonus: For all its space cowboy action, “Star Trek” was remarkably tolerant and progressive in its attitude toward humanity (aliens included) and the dignity of individuals. Why else would the show’s Federation have the Prime Directive, forbidding interference in less-developed cultures? (Okay, so they skirted that directive a few times. Or more than a few.)

(Some of) what makes “Lost in Space” far from great:

1. They bungled a good premise. A space-faring version of “Swiss Family Robinson,” the series could have shown in realistic (even for 1960s TV) manner the dynamics of a family separated from society and fending for itself. But except for a few episodes from the first, more serious season, the show lived firmly in the land of campy entertainment.

2. They let one character run away with the show. Not until Fonzie stole “Happy Days” a decade later did one character — Dr. Zachary Smith, a stowaway on the Jupiter 2 spacecraft — so come to dominate a series, to its detriment. Jonathan Harris — an enjoyable character actor — became more and more the central figure and the other characters faded into the (chintzy) background.

3. The other characters were one-dimensional. The team’s leader and literal father figure, John Robinson, was square and boring. Wife Maureen was usually inside puttering in the kitchen. Major Don West walked into camera range, threatened or insulted Smith, then stalked away.

4. The storylines. Stuck on a random planet for the first season (and then another for the second), the plots usually involved some improbable menace showing up, scaring Dr. Smith, threatening the Robinsons and then being defeated. Yawn.

5. The budget/costumes/effects. Say what you will about the limitations of “Star Trek,” but “Lost in Space” reached the depths of “The Great Vegetable Rebellion,” with actor Stanley Adams in a carrot suit.

Case closed.

More next time.