Every season on "24," a subplot shows up that suggests the writer-producers have run off the rails.  Eventually, the show rights itself; in the meantime, it gets more and more difficult to suspend disbelief and go along on the bumpy ride.

In season one of the serialized Fox action drama, the silly subplot was Jack's wife's amnesia, which had her, like the writers, wandering aimlessly until her senses were regained. Last season, it was the wild cougar, which chased Jack's daughter for a few pointless episodes.

As we reach the midpoint of season three with tonight's episode at 9, the absurd and distracting subplot concerns Chloe's baby.

When "24" is firing on all cylinders, it's terrific. The show always starts with a burst and a blast, and finishes the same way; it's the middle ground that's so treacherous.

The midpoint this season has been aided by the return of two of the strongest female characters from season one: Penny Johnson Jerald as President Palmer's ruthless ex-wife, Sherry, and Sarah Clarke's Nina Myers, who killed Jack's wife to end season one on an unexpected note. The only femme fatale missing is the assassin Mandy, played by Mia Kirshner - who's busy right now flirting with bisexuality on Showtime's "The L Word."

These actresses, like Kiefer Sutherland's leading man, Jack, are tough cookies - formidable adversaries who could well be the heroes, or villains, of their own series. In tonight's episode, Nina and Jack resume their ballet of mutual attraction, repugnance and wariness, and she gives as good as she gets. Sherry, too, springs into action like a coiled snake, and is just as potentially lethal.

Yet while the acting is strong, the show's structure has become, through familiarity, temporarily formulaic. It's a lot to swallow that Jack could have done everything he did to ingratiate himself with the villainous Salazars as a rogue agent, working with two of his peers but with no higher authority. It's harder to accept that these ruthless men, time and again, can be swayed by Jack's last-minute pleas for reason, patience, fidelity or compassion.

But as "24" spends more time on the child-care issue of the baby brought into the Counterterrorism Unit by Chloe (Mary Lynn Rajskub), and forces Jack's daughter Kim (Elisha Cuthbert) to take the child under her wing, the show slips treacherously. The baby may as well be a cougar in swaddling clothes. It's that out of place and that laughable in this otherwise serious series.

The bigger question is how to keep surprising viewers when they're waiting for and trying to anticipate every twist and turn.

The answer is simple: Without warning, and without letting anything leak to the press, put Jack into some deadly peril, and kill him. Only this time, don't bring him back to life. Let the clock keep running along with the plot, but with a new hero or heroine to pick up the slack.

It may be throwing out the baby with the bathwater, but in this case, that's an added plus.