Susan visits and Lois finds out that Susan would die unless she would take Lois's kidney. This is something she would rather prefer, still blaming Lois for stealing Hal away from her. Malcolm and Reese take their new '69 Mustang out for a party. Francis is held hostage by a group of Girl Scouts.
Lois' younger sister, Susan, comes for a visit with gifts for the boys, but Lois is suspicious and this causes a fight between them. Hal later bans the gifts Susan gave them, but won't tell the boys why, among with Hal secretly taking out the engine for the car.
Meanwhile to make extra money, Francis volunteers himself to be a leader for a group of Girl Scouts in the desert, but finds himself over his head when the scouts have a different opinion of fun.
Back at the house, Susan tells the boys why Lois had barred them from the generous gifts she gave them. She claims her belief that the reason their mother acts like this is because she is jealous of her and has to be a control freak to keep herself ahead. Susan mentions it because she was higher class and talented, who can amount to anything. Whereas, everyone saw Lois as a pushy control freak who was untalented and classless. Thus, it was why no one liked being around her. Susan reveals that the only person who was good for Lois was her ex-boyfriend from their time in college, Matt, and she made her biggest mistake in letting him go. Susan also mentions that she used to date Hal and both his family along with Victor and Ida wanted them to marry years ago. She reveals she is still angry with Lois for stealing Hal away from her and the family she should've had with him that lead to her unmarried status. Lois acknowledges the truth, but refused to apologize for her actions because she loves Hal more than Susan ever will. She states that nothing their parents and Hal's family can do to change that, which leads to another fight.
Later on, Dewey takes back his cotton candy machine, but goes overboard with his creations until he gets trapped within it. He would eventually eat his way out of his own trap.
Lois gets a call from a doctor and discovers that Susan is dying from kidney cancer. She never told them this as a result due to the friction between the sisters. Lois secretly goes to Susan's doctor to get herself tested and is found to be a match.
Malcolm and Reese takes back the car from the garage. Reese has the Mustang towed onto a cliff, then leads it down for a big speed and they drive to a keg party, but as the police arrive, they block other people's cars and all are arrested(presumably explained why the kids stopped inviting Reese and Malcolm to their gathering later on).
Susan goes to the hospital to discovered what Lois has done and is angry with her for going with the surgery. She explains that it's a pathetic attempt on Lois' part to try and stay ahead of her. The doctors tries to convince Susan to take the kidney, but it falls on deaf ears. She announces she'd rather die than to accept a kidney from a sister who willingly steal the love of her life away from her. Lois asks her to take the kidney from her because despite their past problems, she does love her. Susan begrudgingly takes the kidney, but still prefers death.
The boys each learned a lesson from all this. Reese and Malcolm being careful in not driving a car without an engine and Dewey cutting back on sweets.
"You see this scar? When I was six, and you were four, dad said we couldn't have dessert, because we were bad. And you dumped his ice cream on his head and said 'well neither do you!' And I laughed so hard I hit my head on the table."