Lena’s ex-boyfriend, Jack Spheer, comes to National City to unveil his big breakthrough in nano-technology, which has the potential to eradicate all diseases. Lena asks Kara to attend Jack’s conference…

Barry and the team go after the Mist, who possesses toxic gas powers, and they reflect on the night Caitlin’s fiancé died. Meanwhile, Henry gets a long-awaited visit from Joe,…