As Barry and the team at S.T.A.R. Labs work to capture Kyle Nimbus (guest star ANTHONY CARRIGAN), a.k.a. The Mist, a dangerous new meta-human with toxic gas powers, they revisit the painful night the particle accelerator exploded and killed Caitlin's fiancé, Ronnie Raymond (recurring guest star ROBBIE AMELL). Meanwhile, Joe decides to finally visit Henry in jail after all these years, but things take a dangerous turn when Kyle shows up looking to punish Joe for arresting him years ago. Meanwhile, Iris and Eddie continue to hide their relationship from Joe.