Dr. Howard Bannister (Ryan O'Neal) is on his way to a convention to try to win a grant; he carries a plaid over-night bag containing experimental rocks. A mysterious stranger at an airport picks up an identical plaid bag containing top secrets. A rich lady carries (you guessed it) a plaid over-night bag full of her jewels. To add to the mix is a walking accident Judy Maxwell (Barbra Streisand) carrying her own plaid bag.
Soon all are inevitability going to converge. Toss in mistaken identities and a bubble bath. You have one of the most memorable comedies.
Lots of fun comedy with most of the clich's, Back and forth in a hallway, Obligatory chase scene, they show their puns, and even a quasi food fight, all over a hand full of bags. At one point Judy hangs around out side Howard's window; one wonders how her towel stays on.
If this ever gets sorted out we are in for a surprise.
If you find the concept of mixed bags funny then you need to also watch Sylvester Stallone in "Oscar" (1991).