When a man is stabbed to death on the eve of Valentine's Day and his girlfriend goes missing, Laura (Debra Messing) immediately notices the similarities to the St. Valentine serial killer murders from the early '90s. The serial killer was caught and died resisting arrest, but after the medical examiner discovers the serial killer's signature - the groom from a wedding cake topper - in the new victim's stomach, Laura and the team realize they have a copycat on their hands - and less than 24 hours to stop him from murdering the missing girlfriend.
Jake (Josh Lucas) calls in a favor with a friend in the D.A.'s office to get Hauser (guest star Enrico Colantoni), who worked on the original case, out on a writ to help them. Hauser insists caught the right guy, but when new DNA evidence isn't a match, Laura realizes there were two murderers - and one is still at large. A candy wrapper (which, of course, Laura recognizes) at the crime scene provides the clue needed to break the case, and the team rushes to the second killer's work place, where they stop him just before he can finish the ritual of the second murder.
Throughout the day, the squad room is rife with Valentine's romance. Meredith (Janina Gavankar) doesn't want Billy (Laz Alonso) to make a public romantic gesture, but as the day goes on, she's obviously having a change of heart. Max (Max Jenkins) receives a mystery bouquet of flowers, and everyone is up in Laura's business, wanting to know what happened with Tony and what that means for her and Jake.
After Billy surprises Meredith with a romantic picnic, and both Max and Hauser tell Laura to follow her heart, it seems Laura will finally confess her feeling to Jake. But when she goes to talk to him, she finds him with his friend from the D.A.'s office: comely A.D.A. Jen Lambert (guest star Jenna Fischer), with whom there are clear romantic sparks. Laura watches, heartbroken, as the two leave for dinner and possibly the start of something new.
