''The Mystery of Irma Vep'' isn't hard to solve. Just rearrange the letters of the last two words and you have a certain supernatural creature that's known to lurk in gothic novels and old horror flicks, among other habitats.
And don't be angry that I've given away the ending, because I won't give away what really counts - the dozens and dozens of laughs that tumble through the Ridiculous Theatrical Company's latest offering, proudly subtitled ''A Penny Dreadful.''
Though its name invites laughter even before the curtain rises, the Ridiculous troupe is a solid and experienced company that has been refining its comic vision for almost 20 years. Its backbone is the versatile Charles Ludlam, a full-blown virtuoso who writes, directs, designs, and stars in all Ridiculous efforts, which arrive onstage at about one show per year.
Going himself one better, he plays no less than three roles in ''Irma Vep,'' which has seven characters and only two actors. The parts he doesn't fill are handled by Everett Quinton, a Ridiculous regular who shares Ludlam's relish for bravura acting, unexpected accents, and costume changes as quick as an eye-blink.
The story begins and ends in a mansion on the moors, and hops to Egypt for the second act. The characters include rich Lady Enid Hillcrest and a servant named Nicodemus Underwood, as well as the mysterious title character and an oversize vampire right out of the ''Nosferatu'' movies.
These figures are parodies, of course, not people. And that goes double for the women, played with flouncing hilarity by the all-male cast. Ludlam and Quinton make them real, though, by charging their performances with all-out energy and conviction - giving each character a personal and emotional life that goes beyond comic devices like a Boris Karloff lisp or a campy outfit. The play is ridiculous, to be sure, but it's sly as well as silly.
It's also great fun to watch Ludlam and Quinton pull off the show's technical feats, whisking from one role to another with great gusto - and poking fun at the conventions of the play itself, as when a Ludlam character points out that another can't enter the room ''for obvious reasons.'' At one point two Ludlam roles do collide, and the result is a half-seen tussle that makes up in invention what it lacks in realism.
The evening's other elements are strong: the costumes by Quinton, the lighting by Lawrence Eichler, the Peter Golub music. ''Irma Vep'' doesn't have the insane inspiration of a first-rate Ludlam classic like ''Bluebeard,'' but it's the next best thing.