Much of “Unleashed” is about confinement: a grown man being kept as a slave, his living quarters a cramped cellar room, his only adornment a dog collar.

So it makes sense that the action highlight is a hand-to-hand fight that takes place in a bathroom no bigger than a closet. Sure, spectacle is nice, but there’s something ingenious about a martial-arts battle with almost no elbow room.

“Unleashed” has a completely mad storyline, courtesy French filmmaker Luc Besson (of “The Fifth Element” fame), who scripted and produced. Like many of Besson’s farfetched notions, it somehow succeeds onscreen because the comic-book premise is taken as perfectly logical.

Jet Li, last seen as the warrior in Zhang Yimou’s hit “Hero,” plays a most peculiar central role, Danny. Since childhood, Danny has been kept as a slave – a dog, really – by a Glasgow gangster (Bob Hoskins).

Danny doesn’t know about the world, he can barely feed himself, and he’s got this painful collar around his neck most of the time. But when his “uncle” takes the collar off, Danny goes crazy, practicing a lethal form of kung fu on the clients who owe Hoskins money.

When Danny finally breaks free, he comes to the attention of a blind piano tuner, played by Morgan Freeman. The latter brings Danny into his home, where his 18-year-old stepdaughter (Kerry Condon) helps civilize the poor, frightened dog-boy.

Of course, this is a movie that needs fights, so Danny won’t enjoy his freedom forever. The past will return, and so will the collar. Just to add an extra layer of misery for Danny, Besson delves into a world of underground death matches, where Danny is expected to kill or be killed. But how can our hero kill after he’s learned to play piano?

The movie, directed by Besson protege Louis Leterrier, invites laughter at various points. You may roll your eyes at Morgan Freeman as a blind piano tuner, or Jet Li learning how to eat ice cream. The implausible business with the dog collar should be a turn-off right away, but this is one of those movies that works if you go with it.

Jet Li is always good at smoldering (he’s pretty good at martial arts, too), but this role calls for a childlike sense of bewilderment. He brings it off in a quirky and original performance; he even seems around 20 years old – which the character would logically be – though the actor himself is over 40.

With Freeman as a wise mentor and Hoskins as a volcanic mobster, the supporting roles are not exactly cast against type. But they both deliver their specialties. And Kerry Condon is a cute find as the girl, a typical Luc Besson character – spunky, innocent, with braces on her teeth – remember Natalie Portman in “The Professional”?

This movie’s touch of madness is frankly just what it needs. But what will Jet Li do next? Fight off assassins inside a cardboard box?

Jet Li stars in “Unleashed.”