A car is pulled out of the river with a dead woman inside. Lassiter and Juliet are investigating the crime scene. Shortly after, Shawn and Gus arrive on the scene. Shawn looks more closely in the car and determines that the dead woman wasn't in the car alone, and therefore hypothesizes that she was murdered.
Gus tells Shawn he has to get to tap class. Lassiter overhears and inquires about it. At the tap class, Gus is surprised to see Lassiter show up. Lassiter insists Gus let him take the class with him as a means to follow through on his therapist's orders to try something new.
Gus is hesitant, but helps Lassie get in the class - which is clearly more advanced than Lassie's current skill level. Lassiter is a terrible tapper, and in turn, embarrasses Gus who had pulled strings to get Lassie a spot in the class. The class ends and Lassie is about to give up on tap all together, but Gus teaches him a few moves. While they are shuffling their feet, they start to talk about the murder case they saw earlier. Lassiter discovers that tapping eases his stress and that he can concentrate on the case more. Lassiter says that he found a bottle of unmarked pills that he sent to lab to have checked out. Lassie explains the pills to Gus and Gus right away says the pills he is referring to are trial sample pills. Lassiter thanks Gus for the help and asks him to tag along on his investigation, as Gus has proven himself useful. Shawn shows up and is immediately jealous that Gus is working with Lassiter on the case and not him. But this gives Shawn a challenge. He bets he can solve the case before Lassie and Gus can. Gus is game, since Lassie gives Gus more respect as a partner than Shawn ever has.
Gus determines that the pills were in fact trial hormone pills, a fertility drug that is experimental, and that the test subject would have had access to them. At the lab the pills came from, Lassiter and Gus question Lillian, the lab tech that was on the job during the time Desiree obtained the pills. But she denies any foul play; she was the only lab tech there. She admits that Desiree was the girlfriend of a lab tech she works with named Ben. Lilian admits that it was she who gave Ben the pills to give to his girlfriend. But Gus figures it out that there must have been a second lab tech Lillian didn't know about because it was a "double blind" clinical trial. After Lillian leaves the room, Shawn shows himself, dressed as a lab tech. He had overheard everything Gus said, and now knows their lead - Ben.
Shawn gets word from Henry that he and Juliet will be working on the case that Lassie and Gus are on - only they will conduct separate investigations. Now the race is on to find Ben. The two teams, Lassie and Gus, and Shawn and Jules both show up at the lead's home at the same time.
As they are in mid-argument, a car rips out of the garage - the teams immediately think it is Ben getting away.
Back at SBPD, Henry reprimands the teams for losing the lead. Lassie and Gus try to tap in synch to help themselves concentrate on the case. They realize that the car they found her body in was shared between Desiree and Ben - but they also determine that the car was rented not owned. Jules and Shawn see them tapping and make fun of them.
Shawn and Jules show up at a science research facility to find another lead. Shawn signs Jules up for a clinical trial to create a diversion, so as to peak into Ben's office. It works and he checks Ben's emails and finds an invoice for flowers he sent to his mother.
Gus and Lassiter are at the car rental place where they discover there was a tracking device on the rental car. They get the tracking I.D., and with that they determine that Ben has motive. But shortly after, Gus and Shawn get a bit of info that leads them to believe that Ben is not guilty. So Shawn and Gus decide to team back up. Now it's Shawn/Gus vs. Jules/Lassie - things are back to normal again. Shawn and Gus arrive at the coroner's office. They want to find everything out about the autopsy of Desiree. Woodson tells them there is a mark on her ankle - looks like freezer burn. He asks, "Why would a drowning victim show no signs of struggle?"
Then Lillian, the first lab tech, shows up at SBPD with some info on their case. Ben's name is officially cleared when she presents them with evidence that Ben was in the office at the time of the murder. Shawn recalls Desiree's mother talking about how Ben had cheated on Desiree. Then Shawn realizes they never found out whom Ben was cheating on Desiree with.
Woody determines that Desiree died from dry drowning - an overdose of anesthesia. Shawn immediately recalls the assistant at the science research - Shawn accuses her of being the other woman and the person that took Desiree out. But the woman says she's just an assistant and doesn't have access to the anesthesia.
Shawn then figures out the case - Lillian the lab tech was the one that killed Ben. They were lab partners - and she fells for him. Ben and Desiree split up - but Ben wanted her back. And Lillian didn't want that to happen. Lillian killed Desiree out of jealousy. Lillian admits it and she is arrested.