"This Is Elvis," the Presley docudrama that opened at four area theaters yesterday, is essentially a compilation film masquerading as biography-fantasy verite', as it were. Filmmakers Malcolm Leo and Andrew Salt put together the wonderful "Heroes of Rock 'n' Roll" for television in 1979, which was their entree with Presley's controversial manager, Col. Tom Parker (who serves as "technical adviser" here).

Since no film exists of Elvis before the age of 21, the filmmakers have combined documentary footage and film excerpts with heavy-handed re-creations using a trio of Elvi; a bloodless first-person narration is provided by yet another Elvis impersonator (Ral Donner) in a style that aims for pathos and insight without achieving either goal. "This Is Elvis" calls itself "the first fully authorized look at the man behind the myth," but one senses the inherent limitations of electronic biography. There are only veiled glimpses of the violent, drug-dependent, emotionally insecure and thoroughly manipulated Presley. One hardly expects such scenes to be captured on film in the first place, but they are also glossed over in the re-creation and in the narrative. This may be Elvis, but Leo and Salt have filmed him tender.

The most revealing shot comes in a late '50s talk show when Elvis, already looking confused and a bit dazed, admits, "everything has happened so fast . . . I can't keep up with everything that's happening." Later, he exhaustedly points out, "the image is one thing, the human being is another." Neither quality is explored adequately.

"This Is Elvis" is at once fascinating and horrifying, a tragedy unfolding. Its major strengths derive from the plentiful kinescopes and newsreel footage as well as the reels of home film the filmmakers uncovered when they went to shoot some scenes in Graceland, Presley's gaudy monument to his fantasies. The scenes farthest back do provide glimpses of the primal rock energy and the incredibly charismatic personality that centered on one of the most beautiful and appealing smiles of our time. Among the many music highlights is Elvis' first TV appearance with a sneering, gum-chewing version of "Shake, Rattle and Roll." There are delicious off-guard moments, particularly a bawdy Elvis trying to atone for a ribald comment by breaking impishly into "What a Friend We Have in Jesus."

There's also the famous army induction that marked the turning point in his creativity; as his hair is being cut, Elvis/Samson can't realize that his power is being shorn forever by a manager who suddenly had too much time to think about career manipulation. One senses the irrevocable downward crossover when a tamed Elvis comes home and mugs on television with Frank Sinatra and his Rat Pack, the two mega-stars trading awful versions of "Witchcraft" and "Love Me Tender" to Nelson Riddle's glossy strings.

The horrifying passages parallel Elvis' inexorable descent into his own private hell, culminating in a performance of "Are You Lonesome Tonight?" six weeks before his death. The last Elvis we see is fat, dripping with sweat (or tears?), incoherent, dazed and stripped of his dignity.

"This Is Elvis" has more than enough moments to satisfy Elvis' fans, but barely enough insight to explain just why this shy country boy became the first spectacular voice and personality in rock 'n' roll or why he left American culture all shook up.