爱好天文的杰弗里（乔恩•海德 Jon Heder 饰）幼年丧父，这令他和母亲简（黛安•基顿 Diane Keaton 饰）之间更加互相依赖对方，已经29岁的杰弗里仍然和母亲住在一起，在父亲好友书店中的工作早已让他厌倦，但习惯了母亲照顾的杰弗里并没有改变生活的想法。简新近结识了一个男友麦特（杰夫•丹尼尔斯 Jeff Daniels 饰），二人的关系迅速发展，麦特不久就侵入了杰弗里的“领地”，此时刚结识了餐厅驻唱姑娘诺拉的杰弗里拒绝了对方的示意，将自己三年未有约会的记录继续了下去，杰弗里自感将要失去母亲，于是向麦特展开了反击，连诺拉也被卷入了这场家庭战争。杰弗里和麦特的对峙逐步升级，而他不期然体会到了和诺拉相处的快乐，诺拉和麦特的出现，令大男孩杰弗里成长了起来……...
A mad scientist grandfather invents a genetic transformation machine and his 4 year old grandson accidentally turns his body into that of a 30 year old man. Baby Bobby escapes and runs around town as his siblings try to catch him before their parents get back from vacation AND before the bad guys get to him first. The machine is worth a lot of money and they only have a day before the machine loses its capabilities to change baby bobby back....
