AT MIDNIGHT on weekends, Source Theater has begun presenting a new rock musical called "The Light," in the hope of imitating the success of long-running midnight movies like "The Rocky Horror Picture Show." But Source has a long way to go before people will line up late to see "The Light."

The playlet's sliver of a plot is about a low- budget cult that dupes its followers into believing they will be removed from this evil earth by redeeming aliens. At the end -- big surprise -- the aliens do show up, performing a ludicrous herky-jerky dance. That's all, folks.

Director Phil Setren has had to add some obvious padding to stretch "The Light" to its current 60 minutes. The show lumps together half-baked allusions to "Close Encounters of the Third Kind," "Liquid Sky," "Rocky Horror" and other alien extravaganzas, with some energetic "Solid Gold"-style dancing by kids clad in outr,e punk garb.

George Singer's skimpy script provides glimmers of wit and moments of camp hilarity, but far too few of each. The cheery synthesized score (on tape) by David Ylvisaker, Doug Miller and Jeff Abrams is burdened with sub- sophomoric lyrics by Scott Gilmore, who also plays a prophetic punk.

The members of Source's ensemble play this stuff straight, as if they mean it, and they would be a pleasure to watch given better material. The one-named Charisma is particularly striking as big bad girl Sadie, revealing a strong singing voice (and in a sheer red dress, a bit too much of her ample flesh), as she cheerfully ignores the demeaning nature of her lines.

This sort of youthful, experimental theater- for-the-fun-of-it is an encouraging sign, and there is clearly an audience for loopy, live late- night entertainment. But this dim "Light" is not worth staying up for.

THE LIGHT -- At Source Warehouse Rep, midnights, Fridays and Saturdays through January 24.