Thanks largely to the dexterity of director John Badham and the irresistibly fresh attractiveness of the young leads, Matthew Broderick and Ally Sheedy, "WarGames" contains enough entertaining suspense and more than enough personality appeal to finesse many of its implausible plot twists.

A diverting update on "Fail Safe," it is modernized by some clever grafting, or perhaps one should say "interfacing," with elements from "The China Syndrome" and "TRON." (The opening image is also a shameful duplication of an image from "Close Encounters of the Third Kind.")

Broderick is cast as a computer whiz kid from Seattle whose zest for video games and sneakiness lead him into accidental communication with a Defense Department supercomputer, WOPR (an acronym for War Operations Planned Response), programmed to analyze potential war situations in deadly strategic earnest. Called David Lightman, the young protagonist is seeking surreptitious access to the master computer of a leading video-game manufacturer, Protovision, and acts lightheaded enough to assume that the clearly ominous list of "new games" that flickers onto the screen of his home terminal can be toyed with. When he asks to play Global Thermonuclear War with the computer and gleefully launches missiles at Las Vegas (big Hollywood joke there) and Seattle while taking the Soviet role, alarms go off in a NORAD command post in the Rockies.

The false nature of the alarm is quickly verified by the startled folks in the war room, but since someone obviously has managed to strike up a conversation with WOPR, which has just been entrusted with expanded fail-safe responsibilities that allow no margin for error, there's obviously a Serious Security Problem. Scared himself, David signs off and wonders if the FBI soon will be knocking at his door. Since the computer keeps calling him back with requests to resume the game, it isn't long before the young intruder is picked up by federal agents and whisked off to the command post for what turns out to be an idiotically imagined line of interrogation.

Right from the start screenwriters Lawrence Lasker and Walter F. Parkes place enough hurdles in the path of believability to oblige spectators to ignore or remove them in order to keep playing an extended game of war nerves with "WarGames." For example, David behaves like an insufferable adolescent when introduced, lewdly insulting a biology teacher in class and using his computer to alter grade records for himself and classmate Jennifer, the Sheedy character. The filmmakers don't seem bothered by the extremely offensive, discreditable, downright stupid aspects of these little pranks, which evidently are calculated to make a brainy boy seem more like a daring juvenile delinquent. The only thing that saves them from disastrous miscalculation on this point is the sheer cuteness of Broderick and Sheedy, who share such dazzling smiles, sunny rapport and innate youthful decency that one simply shrugs off the devious exaggerations as so much junky screenwriting baggage.

The performers make it relatively easy to ignore the absurd blemishes on their essentially spotless teen-age characters. It's more difficult to duck the monkey wrench thrown into the plot by pretending that no one in authority would believe David's explanation of how he penetrated the WOPR system. In fact, the prevalent nonsensical opinion holds that he's probably a Soviet agent (this kid?!) in order to cook up a wild goose chase that leads to a locale called Goose Island, appropriately enough, but must obviously double back to the war room.

It's the movie's most impressive and probably most expensive setting, a vast complex of consoles and giant display screens ingeniously designed by Angelo Graham and his staff to suggest a majestic video game arcade. Where else could David demonstrate his smarts decisively except in this particular "game room" when the great computer seems to be out of control and the existence of the human race supposedly depends on his ability to con or reason it out of triggering Big Bang II? The events trumped up to spirit him out of the NORAD bunker and then get him back again in time for the final countdown amount to nothing but superfluous and transparent delaying actions.

If you can overlook or excuse the movie's dependence on ill-conceived and often unnecessary subterfuges, "WarGames" probably can be relied on for absorbing entertainment. WARGAMES

Directed by John Badham; written by Lawrence Lasker and Walter F. Parkes; director of photography, William A. Fraker, A.S.C.; production designer, Angelo P. Graham; edited by Tom Rolf, A.C.E.; music by Arthur B. Rubinstein; executive producer, Leonard Goldberg; produced by Harold Schneider in association with Sherwood Productions. Distributed by MGM/UA Entertainment Co. Rated PG. THE CAST David....Matthew Broderick McKittrick....Dabney Coleman Falken....John Wood Jennifer....Ally Sheedy