Tammy is back from rehab, and Amy makes a decision regarding her sister.

Amy is upset that Tammy's new nurse isn't pushing her enough; Tammy has started slacking, and the family worries about what could be the cause; Chris and Tammy clash when he confronts her about the new man in her life.

Amy must confront demons from her past, and Tammy admits to cheat days.

Chris comes down on Amy for enabling Tammy.

Tammy must see a new therapist, but things don't go the way she'd hoped; Amy wants to lose her excess skin, but she gets bad news at the plastic surgeon's office; Chris faces the scale; if he's below 400 pounds, his surgery is on.

Chris faces the ultimate weigh-in to get approved for bariatric surgery. With Tammy's weight at a standstill, Amy and Michael make a life-altering decision. The family's fears about Tammy's choice in partners are realized.

While Chris makes progress after his surgery, Tammy gets discouraging news. Amy plans a unique bash for Tammy's 35th birthday. The family excitedly plans their first vacation together with all five siblings, but concerns rise about Tammy's mobility.

The family vacation starts off terribly when Tammy refuses to enter the cabin. Chris hopes he's lost enough weight to fulfill a life-long dream. A last attempt to save the vacation ends in another confrontation. The siblings discover Tammy's secret.

After a rift on the family vacation, Amy returns home and drops a bomb on Tammy. While Amy focuses on her new life, Tammy makes a decision that takes her even further away from her weight-loss goals.

Amy and Michael finally move out of the duplex, while Tammy's partying phase takes a turn for the worse. Amanda tries to get through to her sister before it's too late, and Amy's in for a rude awakening at her annual check up with Dr. Procter.

Amy sees a paediatric dietitian and faces some difficult truths about Gage's diet. Dr. Smith makes a house call and confronts Tammy about her partying lifestyle. While on the road with Chris, Tammy's health hits a dangerous low point.