From An Endless Story:

Shibuya Hiromi's mother eloped with Ootsuka Junko's father, who lived in the same apartment building!? The two were initially at each other's throats, however, realizing their circumstances, both decide instead to take the motto, "Become more happy than those two that left!!" and tackle their love lives aggressively. Both continue on their record of heartbreaks, until Ootsuka finally gets a boyfriend!?