That was the episode Shatner was meant to be in.
Kirk was meant to have been quantum duplicated when they beamed him back to the enterprise, like what happened with Riker, so one got saved, and one went to the mirror universe with the defiant.
We would have got to see a Kirk who was alive in proper star trek canon, potentially not being killed off either, but Shatner wanted too much money, so he never appeared in it.
Ironically they are saying the reason he isnt in the new film is because he is dead in canon, so if he had just made that appearence he might have been in the film after all.
Still he's had emmeys, golden globes, and sang common people since then, so its swings and roundabouts