Carly Spencer was Sam's second love interest. After Mikaela Banes dumps him and he gets the Medal of Freedom from the President, Sam meets Carly, a beautiful, young British woman, and is attracted to her instantly and begins dating her. The two move in together into Carly's Apartment with Sam's dog and pet ex-Decepticons Wheelie and Brains. Carly gets a job as an assistant curator for Dylan Gould, caring for his car collection. Carly, calling Sam her "hero" and wearing nothing but one of his shirts and a pair of panties, wakes him up for his job interview with a stuffed toy for him — a lucky bunny. Later, she is disgusted by Brains when she catches him looking up her shirt and going through her underwear drawer.
Sam visits Carly at work and meets Gould, and is surprised and feels slightly threatened on how close he is with her becoming jealous of Carly's friendship with him, especially as he calls her "duchess". Carly is annoyed by Sam's jealousy and tells him off for it.
After Sam gets a job, Carly, using a new sports car that Dylan gave her— a Platinum Mercedes Sls, visits Sam at work and brings him twizzlers. Sam is annoyed that Carly got a $200,000 car. Carly meets Sam's boss, Bruce Brazos, who checks her out and she compliments him on his "smashing head of hair." She also asks Sam to a party Gould is holding.
After Sam is attacked by Laserbeak at work, Sam takes Carly, Wheelie and Brains to NEST base to talk to the Autobots. Carly is unconvinced they are at NEST and tries to calm an annoyed Sam down and then the soldiers after they detect Wheelie and Brains. Carly and Sam are led into the base after the situation is defused by the arrival of Bumblebee. Charlotte Mearing is annoyed with Carly's presence on the base, but allows her to witness the revival of Sentinel Prime. When Mearing dismisses Sam's actions in the war, an annoyed Carly backs him up and convinces him to leave, saying that he'd done what he'd came to do.
When Carly comes home from work to prepare for the party, she is annoyed to find Sam working with Wheelie, Brains, Seymour Simmons and Dutch to figure out the Decepticons plot. She tries to convince Sam that the Autobots don't need him and to come with her to the party and is furious that he refuses. Ripping a leg off the bunny so Sam has his lucky rabbit's foot, Carly leaves.
After Sentinel betrays the Autobots and murders Ironhide, Sam comes to Gould's party to retrieve Carly and convinces her to come with him. As they leave, Gould reveals himself to be a Decepticon collaborator and the two rush to Carly's car to escape, but it reveals the car is actually Soundwave and holds Carly captive inside himself to force Sam to cooperate and find out the Autobots' plans.
Carly flies to Chicago with Gould and is taken to the building where the space bridge pillars are being set up. She is shocked to see Sentinel there, not aware of his betrayal. Gould explains the Decepticons plans to bring Cybetron to Earth and takes her back to his penthouse in Trump Tower. There, Carly watches through a telescope as Sentinel shows Megatron his place. Eventually, Sam shows up to rescue her, but is flung out a window by Laserbeak. Carly is devastated by his apparent death before relieved that he survives thanks to the Decepticon fighter piloted by Bumblebee. Carly quickly breaks free and enters the fighter's cockpit as it starts to crash and Sam fights Laserbeak. Sam and Bumblebee kill Laserbeak before the fighter crashes, but Carly is unharmed. Using a crashed UAV, Carly relays the Decepticons plans to NEST.
When Sam comes up with the plan to shoot down the Control Pillar with a rocket launcher, Carly accompanies Sam, Epps and his team to the building. She points out which pillar it is and is forced to hide as a Decepticon enters the building before forcing to jump out the windows to escape. Carly slides down the building uncontrollably, but falls through to another floor when the soldiers shoot out windows. When the Driller attacks the building, Carly nearly falls to her death again, but is caught by Sam, who lowers her to a nearby fire escape. Luckily, the group is saved by Optimus Prime who kills the Driller.
Upon reaching the street, the group is attacked by Starscream. Carly and Sam end up separated from the rest of the men and are forced on the run. Realizing that Starscream is after him alone, Sam hides Carly and battles Starscream single-handedly. Carly quickly alerts NEST when they arrive and they start to distract Starscream, allowing Sam to kill him with a small bomb given to him from Que.
After linking up with Epps again, Sam and Carly go to where Bumblebee and three other Autobots are captured — only to watch in horror as the Decepticons execute Que. As Bumblebee is next, Sam tries to help him while Carly tries to hold him back, which works when his second boom stick shorts out. Carly and Sam can only watch in helpless as their friend is nearly executed before a distraction by Wheelie and Brains. Carly watches as Bumblebee kills Soundwave.
Carly rides with Sam and Bumblebee to where the Autobots and humans are battling Sentinel and is forced to watch as Sam heads off to stop anyone from reactivating the Control Pillar. Spotting Megatron nearby and remembering his confrontation with Sentinel, Carly confronts him with the fact that he is no longer the Decepticon leader and will end up being nothing more than "Sentinel's bitch." Megatron gets enraged and wants to kill her, but realizing she's right, heads off to help Optimus.
After Optimus kills Sentinel and Megatron while Sam kills Gould and Bumblebee destroys the Control Pillar, deactivating the space bridge, Carly kisses Sam and they admit they love each other finally. Bumblebee, trying to help, drops a bunch of gaskets to use as rings trying to get Sam to propose to Carly, though Sam says that's taking it too fast but Carly is amused and says she loves Bumblebee.