A Marine Corps judge handed Cpl. Lindsey Scott a major setback yesterday, ruling that defense lawyers cannot show the jury a videotape that purports to prove that Scott could not have abducted, raped, sodomized and tried to murder a woman at Quantico Marine Corps base in 1983.

In his second such decision in two days, Judge Eligah D. Clark said that the videotape "has no probative value" and posed the "possibility of confusing the issues that are before the court."

The judge's ruling came after Scott's attorneys were left reeling and confused by the testimony of a federal fish and game agent, who essentially contradicted a premise upon which the videotape was based. The attorneys had expected the agent's testimony to support the validity of the videotape.

Two days ago, Clark barred the testimony of another defense witness who would have told the jury that the woman's identification of Scott as her attacker was tainted by shoddy investigatory techniques. Although the defense in Scott's court-martial started its case Monday, the jury of seven Marine officers has yet to hear a single defense witness.

Scott's lawyers still intend to present an alibi defense based on the testimony of a former Zayre store detective, expected on the stand late this week or early next week, who says she saw Scott at the time of the attack miles from the military base.

The videotape at issue yesterday tries to trace the route taken by the woman and her assailant on the night of April 20, 1983, when she said she was tricked into accepting a ride from a man who identified himself as a military investigator and told her that her husband had been injured in an accident. The woman has testified that she was driven to a remote wooded area, where she was sexually abused and her throat was cut.

Although the scene of the crime has never been found, a private investigator who prepared the videotape for the defense testified yesterday that he used the woman's own description of roads and landmarks that she remembered to reconstruct and film the route on which she was driven and the route that she walked after the attack. The woman eventually flagged down a passing car, which took her to a medical unit on the base.

Adding together the estimated elapsed time of the drive (25 minutes), the assault (21 minutes), the woman's walk (30 minutes) and the drive to the medical unit (eight minutes), investigator Max Morgan Cherry said the woman must have been abducted at least an hour and 24 minutes before she was first treated, at 9:20 p.m.

As Scott's lawyers stressed, that means the woman would have been abducted no later than 7:56 p.m. -- about five minutes before Scott, a military policeman, was seen at military police headquarters at Quantico.

The woman, who has identified Scott as her attacker, testified that he picked her up between 8:15 p.m. and 8:20 p.m.

Arguing that the judge should allow the jury to see the videotape, defense attorney John F. Leino said, "We can establish certain minimum {times} to indicate that within the prosecution's case, there is a good likelihood that {Scott} is provided with an alibi."

At one point in his argument, Leino practically entreated the judge to allow the jury to see the videotape, even if the jury "may laugh us out of the court."

"That is so significant," he said. "We have tried to be reasonable . . . . We've done everything we could."

But Capt. Steve Hinkle, the assistant military prosecutor, ridiculed the claims of the defense's videotape. Throughout the six hours of testimony yesterday, he stressed that the videotape was no better than a guess -- a guess at the location of the crime scene, a guess at the route followed by the assailant and a guess at the route the woman walked before she flagged down a car.

In questioning Cherry, Hinkle suggested that the private investigator's videotape selectively followed the woman's testimony.

The videotape and private investigator Cherry's testimony, Hinkle said, "fails to establish anything. We don't know where the crime scene is -- that's the bottom line."

Prosecutors were also helped by federal fish and game agent Mac Garner Jr., who testified that one of the main roads identified in Cherry's videotape was blocked by two locked gates on the night of the attack in 1983. Scott's lawyers said Garner had told them earlier that the gates were generally kept open in 1983.

The testimony of Garner, who was in charge of the gates in 1983, seemed to call into question the already shaky validity of Cherry's assertion that the videotape represented the route taken by the woman and her assailant the night of the attack.

Pressed on the issue by Capt. W. Brennan Lynch, Scott's Marine defense lawyer, Garner said Lynch had misunderstood him. "I explained to you in great detail the gates were locked 99.9 percent of the time," Garner insisted.

At one point, Lynch, his jaw tightening in apparent anger, simply stopped asking questions, and stood at the lectern staring at Garner.

Judge Clark ruled immediately upon the conclusion of the argument by the opposing lawyers. Noting that his high school geometry teacher would be proud, Clark recalled the formulaic relationship of time, rate and distance.

If you know any two, the judge said, you can compute the third. But, he added, "Here we have two unknowns: We don't know the distance and we don't know the rate."

He agreed with prosecutors that the videotape was, at best, speculative. "Mr. Cherry has no idea except vaguely as to which route {the woman and her assailant} took, and only a vague idea as to where they arrived at. These unknowns will not give you a known."

After the ruling, Leino acknowledged, "It would've been a significant advantage to have the videotape" shown to the jury.