Guy Ritchie's sometimes entertaining, mostly uneven reboot of Sherlock Holmes drops us into the universe of Sir Arthur Conan Doyle's famed detective Holmes (Robert Downey Jr.) and his trusted friend Dr. Watson (Jude Law) with little in the way of explanation.

When we meet them, the two men have just solved another crime that long eluded Scotland Yard. But Watson is about to marry, and the longtime partners in crime solving are getting ready to part ways.

In an era where most movies based on well-established properties, such as "Transformers" or "Star Trek," get bogged down in endless, mind-numbing exposition, it's refreshing to find one that takes into account its audience's familiarity with a famous character.

In "Sherlock Holmes," though, the lack of back story isn't entirely intentional. Neither Ritchie nor the four writers to whom the screenplay is credited seem to have figured out who this Holmes is or what's driving him.

The movie, too, suffers from a lack of refinement. Whether this "Sherlock Holmes" is supposed to be an old-fashioned detective yarn, a modern-day action thriller or a postmodern romp is anyone's guess.

The story focuses on an evildoer named Lord Blackwood (Mark Strong), thought to have been captured and killed. Mysteriously, he rises from the grave and begins murdering people across London. For the first third of the film, though, Ritchie mostly just luxuriates in the eccentricities of his main characters.

As played with the same foppish, elusive charm he brought to Charlie Chaplin, Downey makes Holmes as physically intimidating as he is emotionally sensitive. In his spare time, he engages in bare-knuckle boxing matches, as if seeking out some sort of spiritual meaning through primal battle. He also seems to have a vague homoerotic attraction for Watson, whose eagerness to sever ties with Holmes is never quite explained.

Except just when the movie might have turned into a strange and entertainingly warped study of a co-dependent professional partnership, it goes in an entirely different (and more generic) direction: Enter Irene Adler (Rachel McAdams), Holmes' former flame, a bit of a con artist who might be working for Lord Blackwood. Cobbling together elements of "The Da Vinci Code" and "National Treasure," the screenwriters take us through a series of familiar paces. The middle section of the movie, especially, bogs down with a mixture of action set pieces and noisy explosions.

Yet "Sherlock Holmes" features any number of terrific flourishes that manage to keep us engaged; Ritchie strikes a perfect balance of the Victorian and digital eras. In the fight scenes, Ritchie slows the images to a crawl, as we watch Holmes imagine the series of punches and blows that will reduce his opponent to his knees — at which point, the images speed up, and each of those movies is executed with elegant precision. Just as clever are the moments in which Holmes explains his elaborate deductions to Watson — a series of rapid-fire flashbacks that piece together a dozen or so bits of evidence. In these passages, Ritchie manages to strike a perfect balance between the Victorian era and the digital era.

If the bulk of the movie isn't nearly so focused, well, it's also far more likable than any of Ritchie's previous pictures ("Snatch," "Swept Away"). The leads look terrific together, and they do their best to resist all stereotypes associated with these stuffy old characters. (To wit: Holmes never says, "Elementary, my dear Watson," and Watson comes off every bit as badass as Holmes.) The climax proves swift and witty, setting us up for many sequels. As the first entry in what's sure to become another billion-dollar franchise, you could do a lot worse.