When fussy Felix (Jack Lemmon) becomes suicidal over his impending divorce, he accepts an offer to move in with his best friend, messy Oscar (Walter Matthau). Felix drives Oscar crazy with his obsession over his soon-to-be ex. Oscar tries to get him out of his funk by arranging a double date with two wacky British neighbors, Cecily (Monica Evans) and Gwendolyn (Carole Shelley). When the plan backfires and Felix grows even more despondent, his friendship with Oscar is put to the test.