It takes a touch of genius to sum up Beckett, the gloomiest of modern playwrights, in a laugh. But Jonathan Harris as the preternaturally servile Clov does just that in the first minute of Splinter Group's heartbreakingly funny Endgame. His laughter, after peering out the window at the desolated earth where he sees "zero, zero, and zero," expresses all the mournful exaltation that gives Beckett's relentlessly bleak masterpiece such cataclysmic sublimity. And Robert Breuler finds within the blind, consumptive, wheelchair-bound petty tyrant of Clov's nemesis Hamm the nearly stilled heart of a poet. Breuler understands that Hamm, who keeps his parents in garbage cans, systematically degrades anyone in his presence for the most pathetic reason: he needs to convince himself that he matters. Desperately clinging to the remains of a once-elevated sensibility, pronouncing a word like "lumbago" as though he were reciting classical verse, Breuler makes the heartless Hamm endearingly human. Under Marc Rosenbush's gentle direction, this Endgame brings the Buckets o' Beckett festival to an exuberant close. Mercury Theater, 3745 N. Southport, 325-1700. May 16 through 19: Thursday-Friday, 8 PM; Saturday, 5 and 8:30 PM; Sunday, 3 and 7 PM. $22-$36.