Though charming and dazzling, show packs a ponderous plot; Mixing music, magic, and 'Merlin' for a Doug Henning spectacular; Merlin Musical comedy starring Doug Henning. Book by Richard Levinson and William Link, songs and incidental music by Elmer Bernstein, lyrics by Don Black. Magic illusions created by Mr. Henning. Directed by Ivan Reitman.

Spectacular magic, magic spectacle, and a touch of delicate charm - such are the pleasures offered by the sumptuous new musical fantasy at the Mark Hellinger Theater. Adventuring back into the mists of legend, ''Merlin'' conjures up the figure of its hero in his nonage, the years when King Arthur's once and future guide was serving his apprenticeship in wizardry. Since the callow but engaging youth is played by Doug Henning, astonished audiences will not be surprised to discover how quickly Merlin masters the tricks of the trade - and then some.

The inspiration for ''Merlin'' is novel and amusing: Integrate not only the great Henning illusions but the amiable Henning persona into an old-fashioned book musical. Wherever possible, the twists and turns of plot devised by Richard Levinson and William Link employ Henning devices for narrative purposes.

For instance, the wicked Queen (Chita Rivera) transforms a black panther into Ariadne (Michelle Nicastro), a designated temptress who betrays her mistress by befriending Merlin. The Queen turns Merlin's tutorial Wizard (Edmund Lyndeck) into a stone statue - a dirty trick that Merlin later turns on the Queen.

Having fallen short of his master's demands, Merlin takes to the road as a wandering conjurer whose wondrous feats include one of the show's most dazzling illusions. Indeed, there is enough magic in ''Merlin'' to delight the eye and baffle the imagination. If only Mr. Henning had been able to make some of the accompanying plot and musical-comedy baggage disappear! Unfortunately, the mixture of fancy and legerdemain grows heavy when it pads out the proceedings with ponderous thematic airs and attempts at conventional buffoonery. Old hands Elmer Bernstein and Don Black collaborated on the music and lyrics for ''Merlin.''

Mr. Henning, as the originator (with Barbara De Angelis), creative force, and star of the show, meets the extraordinary demands of the extravaganza with a smiling diffidence that magnifies the impact of his illusions. His Merlin is the kind of ingenious lad to whom 1980s video whiz kids can relate.

Miss Rivera is as wicked a witch as anyone could wish for, whether maliciously scheming, defiantly singing, or furiously dancing the Christopher Chadman-Billy Wilson choreography. The touch of charm mentioned at the outset of this report is embodied in the exquisite Rebecca Wright as Philomena, the balletic unicorn who mimes her good advice to the sometimes unheedful Merlin. Mr. Lyndeck is in splendid baritone as the avuncular Wizard. The other principals include George Lee Andrews (Old Merlin) and Nathan Lane (a goofy princeling who would not be king).

''Merlin'' has been lavishly designed in the best Broadway medieval-modern tradition by Robin Wagner (scenery), Theoni V. Aldredge (costumes), and Tharon Musser (lighting). It is a tribute to the visual effects that one can seldom tell where Mr. Henning's magic leaves off and that of his collaborators begins.