Not all sidekicks are created equal. Some eventually prove so popular or colorful that they eventually get their own solo spotlight, while others get stuck in the shadows of their famous partner. After a few decades, Robin got his own super-team and then his own title separate from what he does with Batman. On the other hand, there haven't been many efforts to detail the fictional standalone adventures of either Tonto, Dr. Watson or Art Garfunkel.

Sam Axe, the boozing, womanizing ex-Navy SEAL pal of "Burn Notice" hero Michael Westen, definitely fits the former category. If you had been at last summer's Comic-Con panel for the show - in which star Jeffrey Donovan stayed home so that Bruce Campbell could soak up his usual adulation from the fanboys and girls - you would think that "Burn Notice" was, in fact, largely about Sam, and that Michael and Fiona were the sidekicks.

It was at that Comic-Con panel that USA announced plans for "Burn Notice: The Fall of Sam Axe," a prequel movie directed by Donovan, that airs Sunday night at 9, helping to bridge the gap between seasons of the popular spy show. The film tells the story of Sam's final SEAL mission in Colombia, and how trying to do the right thing turned him from a career military man into an unemployed Miami lounge lizard.

Sam's a fine character within the context of regular "Burn Notice" episodes, and Campbell has spent the last 25 years proving his leading man bonafides - usually in micro-budget, straight-to-video trash, but occasionally in something with real staying power like "Army of Darkness." So with him in the lead role for once, with "Burn Notice" creator Matt Nix writing the script, and Donovan (who did a good job directing a recent episode of the series) behind the camera, "The Fall of Sam Axe" should be a blast, right?

Well, yes and (unfortunately mostly) no.

Campbell's got the charisma and self-deprecating sense of humor to carry a whole film. We know that. But the Sam presented here isn't nearly as much fun as Campbell gets to be on the show. "Burn Notice" regularly treads a careful line between making him seem like a clown but also competent and loyal enough that Michael would repeately entrust his life to the guy. It's a tricky balance, but Campbell sells it. Here, Nix seems to want to treat Sam just a hair more seriously, and it throws that balance off.

Of course, that balance also relies on giving Campbell a bunch of strong actors playing well-rounded characters to play off of. Working without Gabrielle Anwar, Sharon Gless and (save for a brief early cameo) Donovan, and instead paired with a pair of flat, one-note characters played by RonReaco Lee and Kiele Sanchez, there's not a lot Campbell can do to keep things interesting.

Nor does it help that the storyline - involving corrupt military officials and an alleged Colombian terrorist group that may be something else entirely - doesn't have enough heft to carry 90 minutes of story (minus commercials). As a very special "Burn Notice" flashback episode of the usual length, it might be fine, but there came a point where I was checking my watch and powering through only out of professional obligation and affection for Campbell.

It's not terrible - just longer and not nearly as good as it could be - and there's some obligatory fan service both to the histories of both Sam Axe and Bruce Campbell. (Sam both uses a chainsaw and says "groovy.") So if you just miss the "Burn Notice" world, and/or are a Campbell die-hard who have Amazon alerts set up for when "Moontrap" and "Maniac Cop 2" are released on Blu-Ray, it's a decent way to pass that time.

But outside of one sequence near the end where Sam gets to enjoy one last moment of uniformed glory (hail to the king, baby!), none of it lives up to the excitement I felt as I sat in that huge Comic-Con ballroom and felt the wave of love for Campbell and Sam crest at this announcement.

Alan Sepinwall may be reached at sepinwall@hitfix.com