The year is 1997. Microsoft is giving away Internet Explorer and killing Netscape Communications. Netscape decides the only way they will survive is by releasing their Netscape browser as open-source in six months, codename Mozilla. To make this happen developers must make thousands of changes and bugfixes. See how they do it.
In the end, Netscape got bought by AOL, effectively dying. Mozilla was re-written once again and became Firefox, which went on to have more users than IE.