Happy Birthday is a RPG Maker mystery game created by DawnSeeker Studio.
When a party for a wealthy family’s ten year old daughter ends with Annabelle, the birthday girl dead, her older sister missing, and the father refusing to call the police, the detective isn’t exactly conventional. She finds herself at the site of the crime – a rented out old mansion – without any trace of memory. It’s Margerie, the landlady, who tells her that she is a private investigator who’s been hired to catch the culprit and bring them to justice.
But how can she solve a murder when she can’t even deduce her own name? Was it one of the party goers who did the young guest of honor in? …Or was it the ghost said to wander the haunted halls? Can she manage to find the missing girl before another tragedy takes place? And will she be able to sleuth out her own memories and the meaning behind their disappearance at the same time?
In the end, the detective finds her old memories and realizes she was Anna all along, who woke up in the house as a spirit. After gathering all the clues, she finds out that her sister was the murderer. Apparently when they were young, Anna accidentally caused the death of their mother and the sister blamed her for it.
If "Turn her in" is chosen after discovering the culprit, the game will show the father crying wondering how her daughter was able to do something like that. He even thinks in leaving alone his daughter in jail, but Anna tells him to help and love her, because she has forgiven her sister.
If "Don't turn her in" is chosen after discovering the culprit, the father will be angry, saying that he will catch the culprit, but Anna calms him. After her father leaves the scene, Anna has a last talk with her sister, scolding her, and telling she will forgive her if she works to make the family happy, her sister apologizes before leaving.
In both endings, after saying goodbye to her father, Margerie (revealed to be a spirit as well) appears and starts explaining how they were able to come back in time to solve her crime, and how after that, both are now ready to move one and reunite with their deceased family, Anna's mother and Margerie daughter.