The Flash battles Captain Cold after the villain is involved in a robbery and plots to steal the Kahndaq Dynasty Diamond.

While Barry strives to become fast enough to defeat Reverse-Flash, Captain Cold returns to Central City with a partner and takes Caitlin hostage to force a confrontation with the Flash.

Felicity and Ray Palmer pay a visit to S.T.A.R. Labs for help getting Ray's ATOM suit to work, and ask Cisco for help. Meanwhile, a young tech expert raises havoc with her insect drones, and Barry asks Ray for help dealing with her.

While Barry tries to find Eddie, a mysterious mind-controlled thief launches attacks on Central City's gold reserves. The team soon discovers that a telepathic gorilla is behind the robberies... and is following Harrison's commands. Meanwhile, Iris confronts Barry, while Harrison prepares to return home.

Barry and the team look for another Firestorm match for Dr. Stein. When the team meets Jefferson "Jax" Jackson, Caitlin has her reservations about whether Jax is the right match for Dr. Stein. Iris surprises Joe while Barry and Patty grow closer.

Zoom arrives back on Earth-1 intent on taking over Central City. Barry and Wells come up with a plan to stop Zoom once and for all but it’s extremely dangerous. Unsure if he should take the risk, Barry reaches out to both fathers for advice. Henry is adamantly opposed to Barry risking his life again but Joe thinks he can handle it which puts the two men at odds with each other. Meanwhile, Cisco is shocked when he vibes the Earth-2 villain Rupture, who happens to be his brother Dante’s doppelganger. Rupture came to this Earth seeking justice for Reverb’s death. Iris decides she’s finally ready to open up to Barry about her feelings for him.