Analyze This was a pleasant surprise, an unlikely comedy hit that both proved Robert De Niro could do comedy - who knew? - and provided him with the perfect partner, the genially wisecracking Billy Crystal.

Analyze That is no surprise, and pleasant is about the most you can say for it.

Reprising their roles as emotionally fragile mob boss Frank Vitti (De Niro) and his reluctant psychiatrist Ben Sobel (Crystal), the pair once again gives us The Godfather with laughing gas, or maybe it's The Sopranos without the pathos (but with the salty language perfectly intact). Maybe it's because we know what to expect, but there's little to Analyze That that goes beyond engaging and too much that doesn't even get that far.

True, the chemistry between De Niro and Crystal is as natural as ever, and it's fun watching them play off each other; I wish they'd tour as a nightclub act. But here, via a plot that takes way too many liberties with itself, director Harold Ramis and his team of screenwriters act as though just having Billy and Bob up there on-screen is enough. It isn't.

Set about two years after the original film, Analyze That has Vitti firmly ensconced as the Big Man in Sing Sing, getting whatever he wants, including - and perhaps most importantly, given Frank's fragile ego - deference. It's not a bad life at all, until some guys start trying to kill him.

So Frank comes down with a sudden attack of schizophrenia, belting out songs from West Side Story (playing both the Richard Beymer and Natalie Wood parts) and then getting real quiet for long periods of time.

Enter Sobel, who's still struggling with the death of his father (a plot point that's supposed to be key, but soon devolves into a single, endless one-liner). After diagnosing Frank as disturbed and getting more so (his examination is one of the movie's best bits), Sobel gets paid for his trouble with a new house guest: The FBI releases Frank into his custody. That solution constitutes a page from the law enforcement manual that none of us has ever seen before, but let's not quibble too much too early.

Of course, the moment he's out of prison, Frank drops the ruse. He's got two things on his mind. Of course, he wants to find out who's trying to off him, but more importantly, he needs to satisfy the libido he's been neglecting for so many months.

And so, within a couple of days of arriving at the Sobel house, he's brought in a tough-talking floozy for some marathon sex. None of this sits well with Sobel's wife, Laura (Lisa Kudrow, underused and under-appreciated), but what can she do?

But here's where the filmmakers stumble the worst, making one of their biggest miscalculations. In Analyze This, Frank Vitti was crass and clueless, but not determinedly so. Whatever the situation, he was always either a big fish in a small pond - which is why he's a good mob boss - or a fish out of water, which makes him a lousy dinner guest at a posh restaurant.

This Frank doesn't seem boorish out of ignorance. He's just boorish, period, self-absorbed in a way that grates rather than amuses. The old Frank had a propensity for saying the wrong things at the wrong times; the new Frank flashes the Sobels' dinner guests. The old Frank was much funnier.

Eventually, Sobel does enough psychoanalyzing to suggest Frank's problems lay in the belief that he disappointed his late father by not becoming a cowboy, and Frank starts getting an inkling of who it is that's out to kill him. For a sly in-joke, note that Cathy Moriarty-Gentile, playing one of the mob bosses who could be wishing Frank ill, was De Niro's Oscar-nominated co-star in the great Raging Bull (when she was known as Cathy Moriarty).

All the while, the FBI (one of the agents is played by Callie Thorne, late of Homicide: Life On the Street) remains on Vitti's case, waiting for him to either slip up and earn a pass back to jail or lead them to some other mob bigwigs.

Everything ends in an absolutely unconvincing plot to knock over a U.S. Treasury van and get a few million bucks' worth of gold bullion. But by the time that starts unfolding, most audiences will have given up trying to follow the movie at all, in favor of simply watching Crystal and De Niro do their schtick.

There are worse reasons to watch a movie. Crystal, as always, is a joy, playing nebbishy with just enough of an edge to keep things interesting. His Sobel even gets to punch out one of the bad guys, a crowd-pleasing moment of the first order.

Still, there should be more than a few funny moments in Analyze That. You'd think unleashing Frank Vitti on a Hollywood set, where the cable series Little Caesar is being filmed, would be a hoot. But the conceit never quite catches fire, despite the best efforts of an unbilled Anthony LaPaglia, who plays the star of the show, an Australian desperate to make things look real.

Maybe the filmmakers should have spent more time here, where Vitti could have played the ultimate fish out of water, in a setting where his coarseness could have proven absolutely endearing. Because every time the movie returns to the theme of who's-after-Frank, it sputters, labors and falls flat.

And that's the problem in a nutshell. Beyond relying on the Crystal-De Niro team, the people responsible for Analyze That didn't even analyze their own script enough to know where the best parts were.

Analyze That

Starring Billy Crystal, Robert De Niro

Directed by Harold Ramis

Released by Warner Bros.

Rated R (Language, sexuality, mob hits)

Time 95 minutes

SUN SCORE * *