Mark Garrison told Prince George's County police last Feb. 6 he had been at home watching "Mission Impossible" the night before when the night manager of the Roy Rogers Restaurant where he worked was stabbed to death.

When detective Mark Nelson of the county homicide unit checked the television listings for the night of Feb. 5, he found that "Mission Impossible" had been preempted by the National Hockey League all star game. i

The detective called Garrison in for questioning, and soon afterward Garrison, 19, of Oxon Hill admitted killing the night manager, Eaker Chern, a 61-year-old retired military officer.

Yesterday, Prince George's Circuit Judge Jacob S. Levin sentenced Garrison to life imprisonment for the first-degree murder of Chern, who was slain after completing his first night working at the Marlow Heights restaurant.

Chern was found sprawled in front of an open safe in the restaurant on the morning of Feb. 6. He had been stabbed repeatedly in the neck and shoulders with a large knife, and $300 was missing from the safe. Footprints in spilled flour indicated he had struggled with his attacker.

Police routinely took statements from all the restaurant employes.

In addition to the "Mission Impossible" discrepancy in Garrison's statement, Nelson recalled earlier this week that he was struck by "some unusual cuts that Garrison had running across the fingers of his right hand."

Garrison told Nelson he cut his hand while working on the fan in his car, but Nelson was skeptical. The detective linked both sides of a large knife and grabbed it. He said he had ink stains precisely where Garrison's hands had been cut.

Nelson said Garrison, who had aroused no suspicion of the first day of questioning, seemed "extremely nervous" when called back in. When Garrison asked if the police had found out who committed the crime, Nelson said he believed that the assailant was a restaurant employe wearing a stocking over his head who accosted Chern when he closed the restaurant.

At that point, Nelson recalled, Garrison interrupted and said: "No. It was a Halloween mask. I did it."

In his subsequent statement to police, Garrison said he needed money to pay off his "marijuana man." He said Chern had called him a "stupid fool" and swore at him during the robbery. In the ensuing scuffle, Garrison's mask was pulled off and he stabbed Chern.

Garrison, who had no previous criminal record, had worked at the restaurant since last August while attending Prince George's Community College.

Yesterday Judge Levin recommended that Garrison be sent to the Patuxent Institution for psychiatric evaluation. Garrison could be eligible for parole in 15 years.