Sydney and Vaughn go undercover as a married couple so as to find a weapon that could bring back the Dark Ages. However, the neighborhood they've infiltrated is not as charming as it seems.

When a mission goes wrong, Sydney's is placed in danger and Marshall is the only one who can save her. Meanwhile, Dixon is exposed to a deadly pathogen released inside APO.

While Sloane awaits the outcome of his sentencing, Sydney's life is put in grave danger when Gordon Dean discovers that Rachel is still alive.

One month after the baby is born, Sydney is taking time off from work. Will is what pulls her back in the field while she's on maternity leave. The enemies realize that the best way to hurt Sydney is through her friends. Anna Espinosa also returns.

Peyton tells Sloane that she has made a deal for the Rambaldi Sphere with an ally in China. Sloane arranges for stolen missiles to be sent to Hong Kong. Sloane's plan is revealed to the APO members, who race to stop him before he can use the Rambaldi Sphere to put the plan in motion. But this all leads to a long awaited confrontation between Sydney and Irina.