The relationship between the male leads in "The In-laws" is that of two fathers whose children had arranged to marry before the families met. It is a situation fraught with upper-middle-class terrors: A successful, respectable dentist has, as he says, spent $6,000 a year to send his daughter to Mount Holyoke, only to have her bring home a second-year Yale Law School student who turns out to be the son of a nut.

The relationship as a comedy team of the two actors who play these fathers, Alan Arkin and Peter Falk, is even more fraught. It's very very very fraught. And also hilarious.

Falk has done a number of parts that are characterized by the trade as "zany," trading on a not-very-startling resemblance to Humphrey Bogart. With his wild-eyed expression and small electric body, he manages to make it seem as if he is always two lively steps ahead of the oversize trench coats he inhabits.

What he has apparently always needed to set off this act was not a collection of matching ersatz 1940s figures, but a big, deliberate, modern, commonsensical figure constantly asking, logically enough, "What are you doing?" and "Why are you doing that?"

They make a marvelous team, Falk and Arkin as, respectively, Vince Ricardo, self-announced C.I.A. agent and father of the bridegroom, and Sheldon Kornpett, D.D.S. and father of the bride.

First we see Ricardo on Kornpett territory - a pre-wedding dinner at which Ricardo makes interesting conversation about his experiences in the Caribbean jungles with eagle-sized tse-tse flies, red tape in the bush and the Guacamole Act of 1917, and then reduces himself to sobs by the eloquence of his bridal toast. Next, Kornpett is dragged into the Ricardo scene of chases and gunfire, steadily retaining a realistic immunity to excitement and glamor, while Ricardo shouts all the government secrets at him in a cafeteria full of board bystanders.

Considering that it's a comic thriller, complete with chase scene - done in a circular fashion across a highway divider - the Andrew Bergman script has beautifully small touches. In one, Ricardo explains with pride that the inscription on his office photograph of the late President Kennedy, "At least we tried," refers to the Bay of Pigs. In another, Kornpett watches glumly while the steward of an agency-employed Chinese airline explains the safety procedures in Chinese and then relaxes for a laugh over a copy of "Better Homes and Gardens."

But those are just details. The themes are whether it's worth risking one's life for the stability of the international monetary system and whether our great democratic opportunity for socil mobility is being undermined by Ivy League mixers.

Two great social issues of our time, the documentation of which is well worth seeing.

THE IN-LAWS - AMC Academy, AMC Skyline, Roth's Tysons Corner, Springfield, Mall, Tenley Circle, Wheaton Plaza, White Flint. CAPTION: Picture, ALAN ARKIN AND PETER FALK FIND LIFE RESEMBLING A BED OF ORANGES IN "THE IN-LAWS."