Dr. Wells' former protégé, Hartley Rathaway, returns to seek revenge on his mentor. Meanwhile, Iris is hired as a cub reporter; Cisco recalls his first day on the job at S.T.A.R. Labs; and Joe asks Eddie for a favor but asks him to keep it a secret from Barry.

Martin and Ronnie are separated, only to realize that they must learn how to rejoin before Eiling hunts them down and kills them... and the Flash. Meanwhile, Joe shows Barry the 3D images from the Nora Allen crime scene, and Barry asks Martin for advice on time travel.

Mark Mardon, having survived his apparent death during the black-matter explosion, returns seeking revenge on Joe for the death of his brother Clyde... and has the same weather-manipulation powers as his deceased sibling.

The Flash learns that Captain Cold and Heat Wave have returned to Central City. This time Snart has brought along his baby sister Lisa to help wreak havoc on the city.