Sid finally has a job, so he will stop doing his celebrated exploits. The job is simple, since you only have to babysit an egg, something that does not seem too complicated.
His friends will help him, since they do not trust him, with all the reason of the world. In spite of that, Sid loses the egg, because there is a villain who is dedicated to look for eggs like the one who is taking care of Sid and steals them.
At that moment, both Sid and his friends go on an egg rescue mission, because Sid had to take care of him and he has to return it safely to whoever entrusted it to him, something that is going to be complicated.
The adventures with this egg make it become the first easter egg in the world, so it establishes a tradition that has reached our days, especially in some countries.