When “Star Trek” guru Gene Roddenberry first pitched the idea of his groundbreaking series to skeptical NBC executives, he described it as “Wagon Train” in space.

Today, even though science fiction has become a grudgingly accepted genre on TV, creators still need to peg their projects to existing shows to keep things simple for network suits. So Trey Callaway must have pitched his new series, “Mercy Point,” to UPN executives as “ER in space.”

“Mercy Point,” which debuts at 9 tonight on UPN, tells about life at the innovative, deep-space hospital of the same name. Its mission (highly unlikely to last five years) is to treat and save the lives of both humans and aliens.

Heading up the cast is the always reliable Joe Morton (“Equal Justice,” “Under One Roof” and other well-crafted ratings failures). Morton plays Dr. Grote Maxwell, Mercy Point’s primary alien physiologist and conscience (for those needing the “ER” analogy, he plays the Anthony Edwards role).

The rest of the cast is largely unknown, and their primary attribute is the ability to look sullen on cue. There’s hints of backstory for each character, but “Mercy Point” is rolling them out slowly, never a great idea in the quick-cancellation world of network TV.

In the early episodes, “Mercy Point” has concentrated on the “ER” portion of its premises, with a focus on medical dilemmas and interpersonal turmoil. The two physician sisters on staff have a bad case of sibling rivalry, bordering on open hostility. One sister has her eye on Maxwell, while the other had a past affair with the hospital’s director of extra-vehicular medicine.

Yes, “Mercy Point” does have some exotic characters. The head nurse is an android, who has to deal with backbiting from her human staff. A visiting, squid-like surgeon drips with condescension (a trait not unknown to human cutters). Yet, for the most part, the series has been dumbed down for general viewers, with the special-fiction elements used mainly as window dressing. (It didn’t help that the review copy, released a week before the premiere, lacked most of its special effects).

“Mercy Point” faces a tough climb in its time slot, squaring off with the critical darling “Felicity,” and the sharp ensemble comedies “Spin City” and “Just Shoot Me.” But as little networks WB and UPN have fewer midseason replacements available, “Mercy Point” is likely to be given time to grow and to strike a better balance among its elements. It’s hardly critical; it’s just not stable yet.