"You. In my universe. Nice to meet you."
Like his prime universe counterpart, Stamets was an astromycologist who experimented with spore drive technology, however, he was working for Emperor Philippa Georgiou. He helped create the super-mycelial reactor which powered the Imperial flagship ISS Charon. However, by this misusing of the technology, he created a disease spreading across the mycelial network. (DIS: "Vaulting Ambition", "What's Past Is Prologue")
At one point, Stamets had supported Gabriel Lorca's coup against the Emperor; however, he betrayed Lorca, which resulted in the rebels being chased down by Imperial forces. (DIS: "What's Past Is Prologue")
As a result of Stamets' experiment with the mycelial network, he, along with his counterpart, became trapped in the mycelial plane, falling into a coma in his lab on the Charon. While there, he met his counterpart and compared notes about his experiments, but without giving away too many details from his side. His counterpart, however, became distracted by a representation of Hugh Culber and he hid himself away from Stamets for a bit. (DIS: "Vaulting Ambition", "What's Past Is Prologue") Due to the actions of his counterpart, Stamets woke up from his coma and was soon captured by Lorca. Forced at gunpoint, Stamets used his technical skills to help Lorca in his attempt to oust Georgiou and take over the Charon. However, once Lorca felt secure he told Stamets that the danger of keeping him alive now outweighed his usefulness. He had him vaporized by Ellen Landry. (DIS: "What's Past Is Prologue")
Alternate timeline and realities
In an alternate timeline experienced by Georgiou via the Guardian of Forever, Stamets attempted to, as a part of Lorca and Burnham's coup, stab the Emperor while she was giving a speech at the commissioning of the Charon in 2255, but, instead, she turned and stabbed him in the neck, leaving him for dead on the stage. (DIS: "Terra Firma, Part 1")