After a few seasons of will-they-won't-they fans finally saw Captain Matt Casey and paramedic-in-charge Sylvie Brett admit their love for each other and embark on a romance.
As season 10 opens, Firehouse 51 remains unaware of their new status and it's unclear how they plan on spilling the beans to their colleagues.
WATCH: Chicago Fire: Casey tells Severide about his kiss with Brett
But showrunner Derek Haas has revealed that the hard work is yet to come, telling HELLO! that although they are both "good people" it's not going to be plain sailing.
Asked what inspired the writers to change their relationship from one of friendship to romance, Derek shared: "They’re both good people and we felt that they realistically would seek each other out."
However, "because of so much history on the show, it had to be a slow burn".
"But it’s not going to be all roses from here either," he added.
Casey and Brett finally made things official
Casey, played by Australian actor Jesse Spencer, has been on the show since season one, where fans saw him lose his fiance in a devastating fire.
He then fell in love and married Gabby Dawson, a fellow paramedic-turned-firefighter, but she left at the end of season seven to volunteer in Puerto Rico, with the pair realizing that despite everything they had been through, they wanted different things.
Casey was married to Gabby Dawson
Brett, played by Kara Killmer, joined the show in season three, arriving in Chicago telling her colleagues that she had left her fiance at the altar after getting cold feet.
However, it turned out he had left her, and they reunited. Brett has also enjoyed romances with Gabby's brother Antonio, CFD Chaplain Kyle Sheffield, and Firehouse 40 Lieutenant Greg Grainger.
One Chicago returns to screens on Wednesday 22 September at 8/7c on NBC
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