Here is the full synopsis courtesy of Marvel:
After discovering an alien city with ties to his resurrection, Coulson and his team destroyed it before the forces of Hydra could claim its secrets, eliminating the villainous Whitehall (Reed Diamond) in the process. But new threats to the world have arisen, including Skye's father, Cal (Kyle McLachlan), who now seeks retribution against Coulson for stealing his revenge against Whitehall; a disturbing alliance between former S.H.I.E.L.D. agent Ward (Brett Dalton) and Agent 33; the enigmatic Raina (Ruth Negga), who struggles with her transformation into something inhuman by the alien Obelisk and seeks vengeance; and Skye (Chloe Bennet), who developed mysterious new powers from the Obelisk but whose lack of experience with her new abilities may threaten the safety of those she loves.I'm excited for the return of this series, even though I'm going to really miss AGENT CARTER being on my television weekly. But the most exciting part is seeing that Dichen Lachman will be back, and that her character now has a name! I love Lachman, and I'm glad they are bringing her back.
Meanwhile, Bobbi (Adrianne Palicki) and Mack (Henry Simmons) begin the next phase of a plan which seems to have grave repercussions for Coulson and his team, who are unaware that there's another mysterious force moving against them. And as Hunter (Nick Blood) is forced to make the biggest choice of his life, Coulson will find his mission threatened by this shocking endgame.
In the midseason premiere, "Aftershocks," Coulson's team must deal with the consequences of their war with Hydra as shocking revelations threaten to tear them apart, and Hydra makes a dangerous move that may involve a traitor in S.H.I.E.L.D.'s midst.
"Marvel's Agents of S.H.I.E.L.D." stars Clark Gregg as Director Phil Coulson, Ming-Na Wen as Agent Melinda May, Brett Dalton as Grant Ward, Chloe Bennet as Agent Skye, Iain De Caestecker as Agent Leo Fitz, Elizabeth Henstridge as Agent Jemma Simmons, Nick Blood as Lance Hunter and Adrianne Palicki as Bobbi Morse.
Guest starring are Henry Simmons as Alphonso "Mack" Mackenzie, Ruth Negga as Raina, Adrian Pasdar as Brigadier General Glenn Talbot, Henry Goodman as Dr. Levy, Kyle Maclachlan as The Doctor, Jamie Harris as Gordon, Simon Kassianides as Sunil Bakshi, Donzaleigh Abernathy as Trip's mom, Philip Labes as young Gordon, Dichen Lachman as Jiaying, Alvin Ing as Yat-Sen, Fred Dryer as Octavian Bloom, Kathryn Leigh-Scott as The Baroness, Joel Polis as The Banker, Kyle David Pierce as accountant, Maz Siam as The Sheikh, McKay Stewart as scientist #1 and David A. Jansen as agent.
"Epidemic" was written by Maurissa Tancharoen & Jed Whedon and directed by Billy Gierhart.
A somewhat off-topic comment: I'm hoping to see Henry Simmons upped to series regular soon since Adrianne Palicki finally was. (While we're at it, can we up Ruth Negga, please?)