Kung (Stephen Chow) is the spoiled youngest brother of three: no job, no education, plenty of money, plenty of girlfriends, and a good home, whom he shares with the two brothers Lo Leung and Fei, his sister-in-law Yinsu (Lo Leung's unattractive wife), and his wise father, Mr. Lo. On his upcoming birthday, as a cruel but well-executed prank, Leung and Fei will feign a lottery win worth $30 million using a useless ticket and a recorded tape bearing the lottery pick from a previous night, all as Kung's birthday present. Excessive greed causes Kung to fall for it, and he immediately (after trying to fool his family so he himself can have all the money) goes out with a selected girlfriend, Gigi, and hits it off at a night club. Instead he runs afoul of Triad member Brother Smartie, who wants a game of dice. Kung easily loses $1 million, as well as $5000 to Gigi, and he relies on his "winnings" in order to pay it all back. When he finally discovers he never won the lottery (much to his shock and dismay), he decides to fake being mentally retarded, a feat which he pulls off quite well. Then he learns he gains more benefits that way, so he chooses to remain mentally ill for the time being. However, he has to learn the true meaning of life and to take what life has to offer but not ask for more.
周星馳主演的三弟終日遊手好閒不務正業，令父親（喬宏飾）和大哥（黃百鳴飾）深感擔憂。此時大哥正打算在長洲開一家餐廳，欲勸三弟過來幫忙。豈料三弟卻變本加厲，終日徘徊在的士高，仗著自己中六合彩頭獎，在的高士不斷請人喝酒，耍闊氣。結果得罪了江湖大佬瀟灑哥（雷宇揚飾），要脅三弟還一大筆賭債，而三弟仗著中獎也不甚擔心。殊不知，六合彩中獎一事，是大哥與二哥聯合起來開三弟的玩笑。使得無法還債的三弟只好裝瘋，一直欺騙家人。
此時二哥（吳鎮宇飾）在長洲結識了一家燒雞店的女少東，小倩（吳倩蓮飾），欲展開一段感情。此時女少東被店舖業主威脅收舖，二哥便把燒雞店的女少東帶回家騙錢，好幫她還店租。然而二哥卻始終忘不了初戀情人甘慕玲（李蕙敏 飾），而甘慕玲惡劣的態度最後還是讓二哥放棄與甘繼續交往。父親與小倩在一段時間的相處下，知道了她的貼心與對二哥的愛，之後父親、大哥決定與女少東一家合作，用計作弄店東，並重開一家新的餐廳。同時間三弟厭倦了欺騙家人的舉動，但一直苦口無言，直到三弟有一天跟父親約見父親舊情人（丁倩飾）後，發現其女小萱（鍾麗緹飾）為了照顧守寡的母親而同樣裝瘋賣傻，兩人便成了朋友。在某天晚上，聽了父親對他的一席既悲痛又疼惜的肺腑之言後，才決定告知實情、面對大家。
