Lucy struggles to get patient Valerie in for surgery when an infection threatens her heart transplant. Carter and Chen disagree over the treatment of an elderly woman and her care-giving daughter. Mark tries to help an abusive gay couple. Carol has trouble adjusting after her return from maternity leave. Dr. Dave goes out of his way to shut an illegal pharmacy down. Luka treats some circus performers. The ER suffers from a water shortage. Benton asks Cleo out.

It's Valentine's Day in the ER. However it proves to be a tough day when Chen and Malucci clash over the treatment of a patient, the parents of two young children die in a car accident, Lucy and Carter treat a law student who she believes may have mental problems, Romano and Elizabeth operate... on his dog, and Abby learns that not all ER cases have happy endings when she treats a dying elderly woman. Mark and Elizabeth go out on a double date with her mother and his father. At the end of the day, the staff party, unaware that Carter and Lucy have both been stabbed by Lucy's patient.