|Season 3, Episode 13|
|Air date||January 31, 2011|
|Written by||David Slack|
|Directed by||Vahan Moosekian|
Killer App is the thirteenth episode of the third season of Lie to Me which originally aired on January 31, 2011.
In this episode The Lightman Group is doing a favor for one of Gillian's therapy patients, Claire. Claire believes that her friend and business partner, Zack, is going to cut her out of their billion-dollar social network company, just like he did to her other friend and business partner, Kyle. Lightman confirms Claire's suspicions when he sees the look of contempt on Zack's face after telling him that she will be fine. Foster goes to check on Claire. Gillian calls Lightman and he rushes to Claire's penthouse apartment to find Gillian covered in Claire's blood. Claire had been attacked and stabbed with a knife. Gillian helped as much as she could, but to no avail, and Claire died. Lightman immediately suspects Zack, but Zack's social network application, "Seekout", and the local cell towers place Kyle at Claire's house at the time of the murder. But, Kyle has an alibi for being at Claire's house at that time, and he insists that a skilled computer hacker like Zach could easily fake his location. Kyle and Claire were also in a romantic relationship. This makes Lightman even more certain of Zack's guilt. Kyle attempts to run a check on his cell phone's operating system to prove Zach hacked into it to fake his location, but right before they get the info they need, a virus from Zack's computer comes and erases everything on Kyle's and Lightman's computers.
Lightman then devises a plan to smoke Zach out and prove he was jealously spying on Kyle and Claire. Detective Wallowski interviews Zach and his lawyer, under the pretence that she needs his help to prove Kyle is guilty. She asks if he knew about their romantic relationship, which he confirms. He tries to play it off as though he only heard through office gossip. However, she catches him in a trap when she asks for proof they were together, claiming Kyle will walk if there is no proof. Zach leaves the meeting and goes to his computer, finding the chat logs he secretly saved between Kyle and Claire, hoping to leak them somehow. However, Cal, Gillian, and Kyle reveal themselves, showing that they knew what he was going to do, and Kyle used a device that mirrored Zach's monitor, thus obtaining proof of what he was doing and that he knew not only were he and Claire lovers, but they were also plotting a takeover. Cal goads Zach further, who gloats "I have a team of lawyers and infinite financial resources. The truth doesn't matter." Cal then shows his hand: He has recorded the conversation, including Zach's implied confession, and uploaded it to Zach's own social network, showing the whole world that he is a murderer.
Cal later goes to visit Zach in jail, telling him he has recommended Zach to go to a mental institution rather than jail. He reasons that jail would only take his freedom, but a mental institution, with the drugs they will give him under Gillian's order, will take his mind.
Cal later goes home and Emily asks him if he loves Gillian to which he answers he does. But Emily thinks he doesn't understand the question, and asks if he loves Gillian in a romantic way to which he also answers yes. Emily asks him why he isn't making a move yet, but he only answers "I don't have an answer for that one, love."