A new relationship started between Denise and Stuart in the second episode of season 12, "The Wedding Gift Wormhole". The couple is seen working together and just hanging out at the comic book store. For their first date, Stuart over does his tanning and ends up orange. Denise doesn't mind though she also like to makes jokes about Stuart's coloring about Nemo, crayons and Chicken ala Orange.
In "The Procreation Calculation", Stuart starts having Denise over to his room in the Wolowitz House and they get romantic to the song "Smooth Operator". Howard and Bernadette aren't happy about this development.
In "The Paintball Scattering", she asks him to move in, but is reluctant. Stuart doesn't want to ruin what they have.
In "The Meteorite Manifestation", Bert wants Raj's help cutting open a meteorite with a band saw. Leonard wants to do it with his laser, but Bert declines. Leonard gets angry that he is left out. After the guys announce that they are cutting open a meteorite, Stuart insists that something bad will come from it. Later, when Stuart and Denise find no one around, they honestly think that something did happen and they are the last couple on earth.
In "The Maternal Conclusion" Bernadette notices that Denise is spending a lot of time at their house even adding items to the shopping list. Stuart tries to have a movie night at Denise's apartment, but they get weirded out by her creepy roommate Mitch. After a rough night spent at the comic book store, she again mentions about him turning down her offer for them to live together. Stuart admits it was a mistake and says he wants to move in. They ask Mitch if he'll move out and he asks Stuart if he loves Denise. They hadn't discussed it, but quickly they tell each other that they're in love.
In "The Stockholm Syndrome", Stuart and Denise are watching the Wolowitz kids while the parents are in Sweden. Halley gets lost during a game of hide and seek, Michael gets a fever and Halley falls down the stairs losing a tooth. Bernadette's parents come over to take charge after that.