本片根据美国作家杰夫·金尼(Jeff Kinney)的同名畅销儿童系列读物改编。第一天升入初中的格雷·郝夫利(扎克瑞·戈登 Zachary Gordon 饰)忐忑不安,拼命想要突出表现成为班级年鉴中的明星。格雷的哥哥罗德里克(戴文·博斯蒂克 Devon Bostick 饰)给了他一堆不能这样也不能那样的忠告,他的好朋友罗利·杰佛森(罗伯特·卡普荣 Robert Capron 饰)却不以为然,继续沿用小学时那一套穿戴、用语、态度,让格雷十分头疼。格雷拒绝了女孩安琪(科洛·莫瑞兹 Chloe Moretz 饰)加入校报的邀请,拉着罗利加入了校安全巡逻队。在逃避万圣节得罪的三个小混混时格雷犯了错误,却又缺乏勇气承认,听任罗利替自己背了黑锅。后续事件的发展越来越糟,格雷在学校的受欢迎度直线掉落,就连他与罗利的友谊都岌岌可危……To Greg Heffley, middle school is the dumbest idea ever invented. It's a place rigged with hundreds of social landmines, not the least of which are morons, wedgies, swirlies, bullies, lunchtime banishment to the cafeteria floor - and a festering piece of cheese with nuclear cooties. To survive the never-ending ordeal and attain the recognition and status he feels he so richly deserves, Greg devises an endless series of can't-miss schemes, all of which, of course, go awry. And he's getting it all down on paper, via a diary - "it's NOT a diary, it's a journal!" Greg insists, preferring the less-sissyfied designation - filled with his opinions, thoughts, tales of family trials and tribulations, and (would-be) schoolyard triumphs. "One day when I'm famous," writes Greg, "I'll have better things to do than answer people's stupid questions all day." So was born the Wimpy Kid's diary.