S4, E6: After months apart, Tom finally reunites with his family and the 2nd Mass. But the joyous reunion is cut short as Lexi undergoes a health crisis tied to her hybrid DNA. The crisis pits family members against one another as they fight over how to deal with Lexi's illness. Anne goes to great lengths to save her daughter. Treva Etienne, Jessy Schram and Robert Sean Leonard guest-star.