Since there is no basis for arguing with the contention of reports like tonight's "20/20" that say America is facing a true drug crisis, perhaps it would be better if cheap and foolish cop shows like the new "Magnum P.I.," premiering on CBS tonight, weren't so quick to make dope an ingredient of the plot.

"P.I." airs at 8 tonight on Channel 9, and the producers will be very lucky if one wag or another doesn't promptly dub the series "Magnum P.U." Tom Selleck plays a very hairy private eye who takes up the mantle dropped, albeit unwillingly, by Jack Lord and "Hawaii Five-O." Our hero lives in the guest house on a Hawaiian estate owned by John Hillerman, an actor to be thankful for even when given a thankless role, of which this one is case-in-point and Exhibit A.

"He was iced with 10 ounces of coke," Selleck says of a Vietnam buddy whose death precipitates the feeble excuse for a mystery plot and for generous ganders at acres of magnificent scenery. To make matters worse, the producers (including Glen A. Larson, maestro of the mundane) thought it would be cute to have Selleck doing a voice-over narration in the style of an old detective movie; "I don't want you to think I was scared or anything" and "We all make mistakes," he tells us viewers.

He also confides that a corpse a few days old smells terrible ("alive he could lead me to LeBoule; dead all he could do was turn my stomach") but it's a note of gut-level realism that doesn't help much. Selleck looks like he just stepped out of an ad for Polo or Chaps or maybe even Jontue. He should step right back in again.