J.J. Abrams’s Alias - which ends its run May 22 - delivered four solid, entertaining seasons of spy drama. Too bad it ran for five.

It’s natural and expected that a show will evolve over time. But when it comes to long-running drama series, most of the time, what we see is not so much an evolution as a downhill slide.

Alias started out as the story of a young CIA agent named Sydney Bristow (Jennifer Garner). As a college student, she had been recruited into SD6, an organization she had been led to understand was a super-secret division of the CIA. In the series pilot, Sydney told her fiancé the nature of her job; in retaliation for this violation of protocol, SD6 director Arvin Sloane (Ron Rifkin) had Sydney’s fiancé killed. Embittered, Sydney ditched SD6, prompting its security team to attempt her assassination. Sydney was rescued by her father, Jack (Victor Garber), who revealed that he was also an SD6 agent, and that SD6 was an enemy of the U.S. government. Sydney took a job with the CIA and returned to SD6 a double agent - just like dad.

With Alias’s fifth season, though, the series started to unravel. Off-screen circumstances - like the 2004 collapse of the real-life love affair between Garner and Michael Vartan (who plays fellow spy Michael Vaughn), as well as Garner’s subsequent maternity (care of new husband Ben Affleck) - conspired against quality drama. Vaughn was killed early this season, amid his engagement to Sydney and the announcement of their imminent parenthood. And with pregnancy impairing Garner’s ability to perform Sydney’s usual ass-kicking stunts, the show wrote in a new female superspy for exactly that purpose.

But, in an interesting turn that should be a lesson for other series, once ABC announced that the current season would be its last, Alias rebounded. Instead of having to devise multiple filler episodes on the way to a season finale that answered a few questions but raised a host of others for a subsequent season, there was a clear end point. Inessential characters were jettisoned, beloved ones have returned; Rifkin’s character, who had been pretending that he was redeemed by a daughter’s love, has gone back to being pure evil. Meanwhile, Alias’s overarching story has recaptured a most welcome momentum.

Admittedly, the show is not leaving the airwaves because its producers were ready to end it; if ABC had been happy with its ratings, Alias would surely have continued for a sixth year. But a finite run has vastly improved the show.

Too often, a hit show, pressured by its network to continue producing episodes, sacrifices quality for longevity. So that a show can be sold into syndication, a story that could have been told over 60-odd instalments is stretched to a hundred. Meanwhile, audiences end up feeling cheated by a show that’s only about two-thirds as good as it could have been.

In some cases, producers prolong the natural run of a series by adding new plots or characters, without considering that a narrative structure can only support so much story before collapsing in on itself. Take The X-Files. It started out as a tale about credulous FBI agent Fox Mulder (David Duchovny) who, having grown up believing his sister Samantha had been abducted by aliens, spent his adulthood investigating paranormal phenomena with sidekick Dana Scully (Gillian Anderson). Some episodes were about Mulder’s ongoing exploration of his sister’s abduction (known to fans as “mytharc” episodes), but the balance of the season was made up of one-off episodes about clones or vampires or talking tattoos (termed “MOTW” episodes by internet wags, for “Monster of the Week”).

The problem was that the more popular The X-Files got, the further it seemed to deviate from a unifying explanation of Samantha’s abduction and its connection to the show’s larger government conspiracy. By the end of the show’s nine-year run, the alien conspiracy encompassed Scully’s cancer, Navajo code-talkers, weaponized bees and all kinds of other mishegoss. When the finale finally came around in May 2002, all hope of sense was irretrievably lost.

A contemporary analogue to The X-Files is J.J. Abrams’s Lost, about a group of people marooned on a spooky island in the Pacific. Since the show became such a hit, Abrams and his producers seem to be rationing the story in smaller and smaller portions, the better to ensure its longevity with viewers and delay the final resolution - namely, whether the passengers of doomed Oceanic Flight 815 ended up on the same plane for a reason. Lost would benefit immensely if its writers could see how much Alias improved when its producers were forced to wrap things up. Lost could end on a high note, leaving viewers excited rather than wearied by years of getting jerked around, as we were by The X-Files.

Shows get stretched beyond reasonable viability by giving each new season its own self-contained story arc. Buffy The Vampire Slayer avoided repeating itself for a while by giving Buffy (Sarah Michelle Gellar) a new “Big Bad” to vanquish each season. While the show offered a mix of mytharc and MOTW episodes, the main business of each season was Buffy figuring out how to defeat this particular villain. It was a fine formula, but after the Season 5 finale - in which Buffy decided the only way to kill evil god Glory was with a blood sacrifice that resulted in her own death - creator Joss Whedon should have noticed that the series had arrived at a logical conclusion. Instead, Buffy hung on for two more seasons of diminishing returns.

Today’s most direct Buffy descendant is Veronica Mars, the story of a female sleuth stamping out evil in the town of Neptune, Calif. It’s still good, for now, largely because Veronica solves one big mystery per season, and these crimes arise naturally through existing characters.

Desperate Housewives, on the other hand, has become an embarrassment. The premise is that even in placid suburbia, apparently model citizens lead complicated, frequently amoral lives. The show’s campy tone is meant to distract the viewer from how hateful everyone who lives in Wisteria Lane is. The series’ catalyst was the suicide of neighbour Mary Alice; once that was explained in the season one finale, the show had to devise a new mystery for season two. Enter Betty Applewhite (Alfre Woodard), who keeps her son inexplicably chained up in her basement dungeon. If this storyline is resolved in this season’s finale, are we to believe that next season, Wisteria Lane will become the adopted home of yet another secretly depraved housewife? And what kind of crazy real estate agent keeps finding these people?

In a similar bit of ad hoc plotting, at the start of its fifth season, Alias seemed to be grooming Rachel Gibson (Rachel Nichols) and Thomas Grace (Balthazar Getty) to be the new Sydney and Vaughn, ostensibly preparing the show for a future without its biggest stars. Instead, ABC forced producers to abandon their plans to keep the moribund show on life support. The upshot was a creative renaissance that rewarded fans for their loyalty and has made the show’s final weeks a genuine pleasure to watch. Alias was good while it lasted. It’s also good that it will soon be over.