In an era where studio blockbusters are becoming increasingly actor-agnostic, Spy is that rare creature: the star vehicle. Sure, there's action, there's comedy, there's lighthearted social commentary, but Spy's success rests entirely on Melissa McCarthy's shoulders.

I went to see Spy almost in spite of McCarthy. I have no problem with her; she simply doesn't compel me to see her work. But I heard good things about the film, and I do drag myself to studio comedies from time to time, usually just to see what all the fuss is about, but sometimes also out of an eagerness to support original material that goes against the grain. Yet five minutes in, I realized that my enjoyment of the movie would depend on how much I like McCarthy's shtick.

Ultimately? I found the actress likable, and Spy enjoyable, but the whole affair disposable.

In a twisty-turny plot, McCarthy plays Susan Cooper, a frumpy CIA agent who's been stuck in the basement, providing in-ear guidance to an archetypically dashing field operative (Jude Law), for a decade. When a Bulgarian villainess (a droll Rose Byrne) kills him, then outs the agency's other top spies, Susan volunteers to finish the job because, as she aptly puts it, she's "invisible". Nobody would think to watch out for her, because nobody ever has.

Liberated male Feig, who's directed McCarthy in female-driven comedies Bridesmaids and The Heat, is making an obvious but salient point here: that in our sexist, looks-obsessed society, a smart, competent woman like Susan may find her talents undervalued if she's not young and pretty as well. (The f-word – fat – is never spoken out loud in Spy, though it's clearly at the heart of anyone's doubts over Susan's abilities.)

There's an infectious spirit of empowerment in Spy, yet there's something weirdly unfinished about the movie.

Chiefly, Susan has no character development. For the first act, she's bumbling and self-deprecating, then all of a sudden she becomes foul-mouthed and intimidating, and stays that way for the rest of the film. I literally felt as though an important scene was cut, one where we see Susan discovering her true self, and realizing that all those years in the CIA basement have made her angry – and that she can channel her anger into getting the job done and earning her colleagues' respect. But this revelation isn't even implied: the instant shift from dorky Susan into ball-busting Susan is purely plot-driven.

This arbitrariness pervades the film. I laughed several times, and found some scenes nicely suspenseful, but Spy's tone is all over the place. Sometimes it's witty, sometimes slapstick-y. Sometimes it's clever, sometimes it goes for cheap shocks. Sometimes the comedy is derived from a situation or character and sometimes it's purely farcical and out of left field.

Feig, McCarthy, et al have delivered a solid B-minus movie. It'll play great on TV. But the more I think about Spy, the more I realize how much potential it had, and how much of that potential it squandered in the name of the quick and easy gag.