Midnight Masquerade is a guided game in The Sims Pet Stories. It starts when Stephen Loyal, an accomplished SimCity chef, who leads a quiet and ordered life in the desert suburb of Mesa Flats has been lately unusually busy getting ready for his cousin's wedding and helping his employer Julianne cater the annual Mesa Flats Midnight Masquerade ball. Little does Stephen know that his life is about to get turned upside down....

While Stephen's cooking his first meal in a new house, his boss Julianne will come asking him to make the salad and the lobster as well as to create the ice sculpture for the Mesa Flats Midnight Masquerade. Soon, his friend, James, comes to greet him and remind that Celeste's wedding is soon to begin. Stephen then changes his outfit into formal one and heads to the chapel. After arriving there, he comes too late, so he apologizes to his cousin. Stephen later meets Gordon Fetching but he dislikes him after Gordon called him Late cousin. Stephen also meets Rachel and after dancing with her he askes her to go on a date but she rejects because she already has plans with Gordon. Celeste says to Stephen that her friend Erin won't be able to take care of her cat, Diva, while she's on honeymoon, so she asks Stephen to do it.

The next day, Erin Davis comes to the Stephen's house bringing him the cat. Stephen thinks Erin is cute and offers her espresso. While they're drinking it, Diva starts to scratch up Stephen's favorite couch and Erin calms Stephen down by saying that Diva's just getting familiar there. She also says that she used to be a veterinarian, so he can trust her.

The next day, Stephen decides to buy a litter box for his cat. While in the store, he sees Gordon flirting with one woman who falls in love with him immediately. After that, Diva comes and scares Gordon, and the woman laughs at him. After Gordon found out that Diva is Stephen's cat, he says to him to keep her away and that he should also stay away from Rachel. Later, James comes and they watch TV where it's said that a con-man was discovered operating in Arbor Falls and it's believed that he may have moved on to Mesa Flats.

Next morning, Stephen decides to work out a bit and then he invites Rachel out. They go to the Club Z where they dance. While Rachel is asking Gordon whom he's gonna invite to the Masquerade, Gordon comes and interrupts them. Stephen then goes home.

Another day, Stephen decides to give attention to Diva so he plays with her a bit. Later, Erin comes to his house and he asks her on a date. They go to Chez Prix, but after eating food they feel sick as well as the whole restaurant. They decide to take photos in the photo booth and head home.

The next day, Stephen, after cleaning his kitchen, prepares salad for the Masquerade but Diva eats it. He goes to Six Points Plaza and buys some groceries. He meets Erin there and asks her to the masquerade and she says she'd love to go with him, but Gordon already asked her.

Next day, Stephen prepares Lobster for the Midnight Masquerade. He also invites James who says they should go to Flat Hat Bar because Stephen's house looks like a petting zoo. After he comes there, they play cards a bit and James tells him that con-man's last name starts with an S which means Gordon is not that guy. While they're talking, Rachel comes and joins the game. She later dances with Stephen and while she's at the ladies' room, Gordon comes and fights with Stephen because he warned him about Rachel. After that, Rachel comes and Gordon asks her if she'd like to go to masquerade with him, but she says that she had already agreed to go with Stephen.

The next day, Rachel comes and invites Stephen to Club Z. After he meets her there, he sees that Gordon is also there with Erin. After Rachel and Stephen sing a karaoke together, Gordon comes and talks to Rachel but she doesn't want to do anything with him. Stephen then decides to kiss Rachel so that Gordon leaves her alone. Erin sees them kissing and asks Stephen if he's dating Rachel now to which he replies 'sort of' which makes Rachel angry and she tells him she'll call tomorrow.

Next day, Stephen invites Erin to Sal's Bratwurst and Culture because he feels bad about what happened at Club Z. He introduces her to his friend Sal, and after they eat they slow dance. Erin tells Stephen that she thinks they are at different places in their lives. Stephen interrupts her and starts to insult Gordon which makes her angry because she just wanted to tell him she's confused, not get him start on a rant about someone she cares for.

Next day, Rachel calls Stephen and ask him if he'd like to help her pick a mask for the Masquerade. While at Six Points Plaza, he sees Erin and buys her roses and apologizes to her. Later, James comes and Stephen tells him that he's in love but not saying with who.

On the Masquerade day, Stephen calls Rachel to see if she's ready and he heads to Amber Hall. After he comes there he puts Gordon's mask on which Gordon removed in order to play music. Stephen then beguiles Gordon's henchman, Anthony Zilch, who falls for his disguise and tells him that everything's in place and that they'll have all the girls' simoleons by morning. Anthony also reveals that Gordon's last name is actually Schickel. Stephen immediately tells that to James who calls the police. After Gordon is announced as the masquerade king, police arrest him so Stephen is chosen instead. Diva also stops Anthony from escaping. Now Stephen must decide who will be his queen: Rachel or Erin?

If the player chooses Rachel, he'll ask her to move in and she'll give him a hot tub as a present. If the player chooses Erin, he'll propose engagement and they'll go on a romantic trip to Simaican Isles. On the end, when Stephen checks his email, he'll see a message from Celeste who says that she's going on a cruise with her husband and that he should keep Diva.