Mark is taken down in a Prolethean shoot-out and Sarah must step in to help. But as Sarah gathers intel that puts her on the trail of the original Castor DNA, a familiar foe is not far behind. While Helena continues to toil in a faraway military compound, it is ultimately Sarah’s fate that will hang in the balance as she tries to save her sister. Meanwhile, Gracie is cast out from the Prolethean fold, and Alison and Donnie come face-to-face with unhappy drug lord, Jason Kellerman.
Alex’s worst nightmare is realized when she gets news that she has to share the Valedictorian title with her number one competitor, Sanjay! Both sets of parents get equally caught up in the rivalry and convince Principal Brown to let them break the tie via a gym grade they both have not completed – a one mile run. Elsewhere, Gloria is studying for her upcoming U.S. citizenship test and a visit from Javier throws her off her game, and Mitch and Cam blindly agree to join a protest against a local restaurant that they are actually secretly excited about.
Mike’s incompetence threatens to ruin Frankie’s Mother’s Day. Mike tells Axl, Sue, and Brick to put some effort into a gift for Frankie. Frankie asks the kids how they would differ from her as parents.
Oliver/Al Sah-him finishes his training and is left with two final tasks to become the next Ra’s al Ghul. Ra’s threatens Nyssa and delivers some shocking news. Meanwhile, Malcolm makes a surprising offer to Team Arrow and Thea goes to see Roy.
Skye is torn between her loyalty to S.H.I.E.L.D. and her connection to the Inhumans as tensions rise between the groups — and Coulson reveals a secret he’s been hiding from even those closest to him.
Barry must deal with the Reverse Flash’s latest threat to a member of the group. To make matter worse, Dr. Wells unleashes Grodd on the city in order to distract Barry and the team. Joe, Barry and Cisco head down into the sewers to catch Grodd but the gorilla quickly gets the upper hand after he kidnaps Joe. Iris and Barry have a heart to heart talk.
When a clever clever con man, Professor DeSalvo, uses his class to cheat the study group, they enlist Jeff to get revenge. The situation goes awry as Britta and Dean Pelton get involved in Jeff and Professor DeSalvo’s scamming competition.
When Sid Ross, the powerful creator of a long-running sketch comedy show is killed, Castle and Beckett are called to investigate. But as they delve into the victim’s life and the colorful world of “Saturday Night Tonight,” they uncover a shocking secret that may explain Sid’s murder.
When a former client calls Alicia after being charged with murder tied to a case from 2009, Alicia reviews the old case, prompting memories of her early days as a lawyer.
The Faith Militant grow increasingly aggressive. Jaime and Bronn head south. Ellaria and the Sand Snakes vow vengeance.