Joe and Cisco go to Starling City to ask Quentin Lance for his help investigating the site of Harrison's car accident 15 years ago and determining what really happened. Meanwhile, Eddie is framed for a shooting by a shapeshifing metahuman who can take on the appearance of anyone he touches.

When Harrison activates the particle accelerator, the team has no choice but to evacuate the five metahuman prisoners. Unfortunately, with time running out, Barry has no choice but to turn to Captain Cold for help. Meanwhile, Eddie escapes but has some surprising news for Iris.

Jay Garrick, a mysterious man from Earth-2, appears at S.T.A.R. Labs with a dire warning about an evil speedster named Zoom, who is set on destroying The Flash. Barry and the team must decide if they can trust this stranger even as they face yet another powerful meta-human. Meanwhile, Joe must deal with a determined officer named Patty Spivot who wants to join his meta-human task force.

Barry learns a new breacher, Dr. Light, has come through the portal and sets off to capture her. Jay tells Barry that Dr. Light was not a threat on Earth-2 and that Barry can reason with her. However, during a fight with The Flash, she blinds him and drops some shocking news about Zoom. Meanwhile, Barry and Patty go out on a date.

Vandal Savage arrives in Central City and sets his sights on Kendra Saunders. After Vandal attacks Kendra and Cisco, they turn to Barry for help. Realizing how dangerous Vandal is, Barry takes Kendra to Star City and asks Oliver and team to hide her until he can figure out how to stop Vandal. However, things quickly go from bad to worse when a man with wings AKA Hawkman shows up and flies off with Kendra. Meanwhile, Harrison develops a serum to make Barry run faster and asks Jay to test it out.

The crossover continues on Arrow S04E08 Legends of Yesterday (II).

When Cisco gets a vibe of Eobard Thawne, Barry and the team don’t believe it. But, after an attack at Mercury Labs, Christina McGee confirms that the Reverse Flash is back. Meanwhile, Iris and Francine share a nice moment that brings Iris closer to her brother, Wally.

On Earth-2, the team races to find Zoom's lair and asks for help from the most unexpected source. Meanwhile, back on Earth-1, Caitlin rushes to perfect Velocity-9 so that Jay can stop the Geomancer.

After the shocking revelation that Zoom is Jay Garrick, Barry is more determined than ever to get back to Earth-2 to stop Zoom forever. Desperate to find a way to increase his speed, Barry decides to travel back in time and masquerade as his earlier self in order to get his arch-nemesis, Dr. Harrison Wells/Eobard Thawne, to teach him how to run faster. However, things don’t go as planned and Barry is stunned to face familiar foe Hartley Rathaway, as well as old friend Eddie Thawne. Iris makes peace with her past in order to embrace her future.

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.