本片是《麦兜故事》系列的第三部，由现实和动画两部分故事组成。现实部分的故事开始于一次“人质挟持事件”，一群持枪匪徒闯入香港中环的国际金融中心（IFC），劫持了一家广告公司的数十名职员，在和警方对峙的关键时刻，匪徒提出要吃外卖盒饭，警方进一步调查发现此案线索涉及“春田花花幼儿园建校五十周年校庆”，而匪徒在与广告公司职员交谈时，发现某个直发披肩女孩（周笔畅）来自著名而神秘的网站春田花花幼儿园，产生好奇。而穿插在现实部分里的动画故事，春田花花幼儿园的小朋友幻想着自己将来快乐简单的理想职业生活，可是长大的他们（黄秋生、郑中基、梁咏琪等），换取温饱的方式多数变成了目标不分的随大流装傻。

在送雞迎狗的大除夕，中環國際金融中心發生脅持人質事件，特別任務連在門外準備拯救人質，而賊人提出的第一個要求是「囉仔記」的窩蛋牛肉飯等外賣飯盒。這要求令警方掌握了重要的線索，原來事件與即將來臨的「春田花花幼稚園50週年」慶典有關。 當年「春田花花幼稚園」的小朋友，畢業後各自在各行各業發展，現在成為了「春田花花同學會」的一份子，但成年人的工作世界竟然同小時候的世界一樣，目標不清的，大家最終期望其實只是「搵兩餐」。

In McDull, the Alumni, our protagonist has grown up. He is no longer the little boy who banters with his mates at school. How he wishes he could just go on bantering all day long with his mates at the renowned Flower on the Spring Field Kindergarten. But that is not to be. Like all grown-ups, he has to grapple with harsh reality. McDull and his mates are scattered all over the place. Each one of them has to find his or her own path. In each of their hearts, they know they have failed. Meanwhile, life goes on in the kindergarten. Someone strums a guitar and the pupils chime in to the song: Puff the magic dragon, lives by the sea… A dragon lives forever, but not so little boys. With hindsight, McDull believes this could well be the maxim of the Flower on the Spring Field Kindergarten alumni.