A toxic gas will be released into the city of Toronto, unless Murdoch can find a way to stop it.
Bay hosts an anti-Valentine's Day party, where Noah and Travis get into a fight. Meanwhile, Angelo and Lana call a temporary truce; and Regina disagrees with the Kennishes on how to deal with a situation concerning Daphne's food truck.
Five-0 investigates the murder of a traditional Hawaiian fighting master. Meanwhile, McGarrett hires a private investigator to follow his mother.
When Jax and Laura-Leigh are caught having sex in SUR's bathroom and Frank insults a customer, Lisa is forced to crack the whip on her staff, resulting in a firing. Stassi thinks her life is back on track when she makes up with her old friend Katie and introduces her new boyfriend Frank to her parents, but when an unexpected argument escalates, Stassi finds herself headed for her second break-up of the summer.
Carrie runs into an old friend at her internship and gets invited to his mother's dinner soiree in Manhattan, but her feelings for Sebastian get in her way, until she see him with another girl. Mouse is excited about getting back together with Seth, but her worries about being bad at sex drive her to ask Walt for help. In order to hang on to a favorite spot at the diner, Maggie takes on Donna LaDonna and her friends. Meanwhile, Tom loses his wedding ring, which makes him realize that Harlan might be right about starting to date again.
Cora brings the giant to Storybrooke, where he unleashes vengeance on the town; Emma and Henry go with Mr. Gold to search for his son; Greg asks Belle what she saw on the night of his accident; Anton the Giant befriends humans.
Harry loses his cool with 3 swimmers that he's forced to rescue just seconds after warning them not to swim in a dangerous area. However, calling them "imbeciles" and "idiots" puts him in hot water.
In Florida having a nice pool is a must. So Vanilla Ice and his crew decide to salvage as much of the existing pool as possible but also kick it up a notch. They bust up the bombed out pool at this house to turn it into a tropical oasis fit for a rock star.
Kim invites her entire family to Miami to compete in a dragon-boat race. At the same time, Kourtney suspects her sister-in-law is pregnant, while Scott has trouble dealing with Bruce's competitive nature.
Carl says his goodbyes; Frank is on babysitting duty; Sheila wants to introduce Jody to toy time.
Bart's dream girl, Mary Spuckler, comes to town, but his failure to pay her enough attention is having a negative impact on their relationship. Meanwhile, Homer once again runs afoul of Marge and lands in the doghouse.
Scott discovers that Spartan has a broken leg and Amy is forced to make the toughest decision of her life.
Matt Damon (as himself) hires the firm to find a charity to enhance his image, but Marty soon realizes he may have to dig deeper than expected in order to keep the actor as a client. Meanwhile, Jeannie finds time to spend with Roscoe; and Tessa grills Monica about their status.
Roger announces that Max Jets, a character who showers the Smith family with money, is making a return; a gold-digging waitress romances Max and threatens the family's inheritance.
Jake's worst fears are realized upon discovery that Leslie has gone missing while working her dangerous undercover assignment, and now he's determined to find her at all costs. Meanwhile, an unlikely and suspicious client enlists Des's PI services.
When a heart-shaped pancake simply doesn't cut it, the kids convince Bob to let them skip school in order to help him find a Valentine's Day gift that will "wow" Linda. Meanwhile, in the spirit of the romantic holiday, Linda sets up a speed-dating event at the restaurant that doesn't go exactly as planned.
The Humping Robot attacks the U.S. Navy in Battlehump; an intervention for M.A.D. Cat; imagine how the Thundercats learn about estrus.
On Valentine's Day, Meg goes on a date with a guy she met online, and Lois and Peter stay in bed all day; Brian is visited by his ex-girlfriends.