Shawn and Gus investigate the murder of Mauricio Sanchez, owner of their favorite taco food truck in Santa Barbara.
The first round of possible suspects they interview are the food truck owners who park their trucks near and next to Mauricio's. Unfortunately, no one is able to provide any useful information regarding the murder. So, in order to get an insider's perspective on what happened, Shawn and Gus go undercover as owners and operators of their own food truck. Lassiter wants nothing to do with the food truck because he deems them unsanitary "roach coaches." In fact, he hates them so much that he recently impounded one.
Shawn and Gus have plans to use to the impounded food truck for when they go undercover.
But before Shawn and Gus leave the SBPD to set up their food truck, they see Marlowe, who is now 8 months pregnant, and learn that because of a heavy workload, she and Lassiter haven't had much time to properly prepare for the baby's arrival. This worries Lassiter and Marlowe, so they do everything they can to make up for lost time; like baby-proofing the house, signing up for parenting classes and carrying around a practice baby.
Shawn and Gus arrive at the same circle of food trucks where Mauricio was found dead. They have their own truck called "Mash & Grab." Menu items like "ice cream jerky" and "pork tarts" are among their specialties. While working the line, Shawn notices a busy Mexican food restaurant across the way. He thinks the owner there could be a suspect because his restaurant was losing profit to Mauricio's taco truck. They call up Lassiter and together the three of them go to question the owner. There, the owner does admit that he and Mauricio were bitter rivals, but says he didn't kill him. Plus, he has an alibi.
Shawn and Gus' next move is to walk around the food truck vicinity handing out free samples because they think it will make it easier to question potential suspects. They meet one guy, Jake, owner of the gym "Handles" for overweight men, across from the food truck circle who isn't pleased that fatty foods are being peddled right outside his business' door. He threatens Shawn and Gus, so they move on.
Back at their food truck, they discover the Health Inspector has made a surprise visit. They receive a 'D' grade and are told they have two days to address numerous violations, or they'll be shut down. Shawn and Gus turn to Henry for help. While at Henry's house, Shawn is hit with the shocking news that Henry is selling the house. Henry explains that he's getting older and it's time for him to downsize. Shawn has a lot of memories in that house so he has trouble accepting it. Henry doesn't have time to help out the guys, so Shawn and Gus are left to their own devices.
That night Shawn and Gus secure a prime parking spot for their food truck so they're the first truck to be up and running for the breakfast rush. Suddenly, a masked man tries to rob them, but thankfully the thief is scared off by a passing car.
The next morning, Shawn and Gus learn from the other truck owners that theft is common in the food truck world, but it hasn't been a problem for them ever since they hired a local gang called "The 108's" for protection. One of the other owners gives more detail. She says that whenever a guy named "Smokey" comes around, she's supposed to give him her share of the protection money. All the of the food truck owners did this, except for Mauricio; he always refused to pay. Shawn then realizes that the black eye and busted lip Mauricio had on his face at the time of his death were given to him before the murder took place.
They go to Smokey's house to get some answers. There, Smokey's mother answers the door and Smokey goes running. The guys catch up to him and discover he's the same guy that tried to rob them the night before. It turns out, Smokey is pretending to be a gang member of the 108's so he can shake down the food trucks for money. Shawn accuses Smokey of killing Mauricio, but he denies it and says he only scared him. He then tells the guys that Mauricio was already engaged in a brawl with a guy in a tie carrying a clipboard before he got there. Shawn remembers that the health inspector they saw earlier was wearing a tie and holding a clipboard. He also remembers seeing bruise marks on his fist. This, paired with the fact that all the food trucks were all given 'A' grades but Mauricio's truck, makes Shawn believe he's the killer.
They questions the health inspector about the fight he and Mauricio had. The health inspector says Mauricio was the one who got aggressive with him after he gave him a 'C' grade. Mauricio thought the health inspector was sabotaging him and his business. The health inspector says the only reason why he was there to inspect Mauricio's truck in the first place was because he received an anonymous tip that there were rodent droppings seen on the floor of it. Given this information, the health inspector is not the murderer, so Shawn and Gus need to search for another lead.
After Henry calls Shawn to tell him he has accepted an offer on the house, Shawn and Gus drive the food truck over there to collect Shawn's stuff. There, they see Henry having a yard sale. Shawn notices Henry is selling his gym equipment from "Handles," then instantly recalls the encounter they had with Jake the owner of Handles. Shawn puts a few clues together and realizes that Jake was likely the guy that gave the health inspector the anonymous tip about Mauricio's truck. Jake wanted the food trucks away from his business, so he did what he felt was necessary. He singled out Mauricio's truck in particular because his truck was the truck always directly parked in front of Handles. But the 'C' grade the health inspector gave Mauricio wasn't enough for Mauricio to leave his food truck spot, so Shawn thinks Jake confronted Mauricio, things got out of hand, and he murdered Mauricio. They go to question Jake, and are shocked to find him dead. Clearly he's not their guy.
They reassess. News of a bank robbery leads Shawn to realize that all the food trucks always park in front of two different banks. Shawn believes that one of the food truck owners was using their food truck as a cover while they planned a way to rob the banks. Shawn gets a tip from McNab that leads him to believe that the person who owned and operated his and Gus' food truck before they turned it into "Mash & Grab" is likely the murderer. The truck was Mexican food themed, so whoever owned it was in direct competition with Mauricio. They go to talk to the original owner of their food truck, and he tells them that he had to give up his food truck business because he ate too much of his own food and he kept failing health inspections, not because he was losing business to Mauricio. He's not Mauricio's murderer. But he gives Shawn and Gus a tip that provides them with another lead. He tells Shawn and Gus that some of his fellow food truckers were also closing down because of similar failures, including the healthy vegan-themed food truck, "In the Vegan-ing." But Shawn says he remembers talking to the owner of the vegan truck just days before; his truck was still up and running just fine. Someone must have put up the money to save the vegan truck and Shawn believes that it was Jake the owner of Handles who did. Both had struggling businesses with similar clientele, so they tried to form a business relationship to help each other out, and when that didn't work, they became partners in crime and robbed banks.
Shawn and Gus head back to the food truck area to confront the vegan truck's owner Vito, and are surprised to see Marlowe there. Marlowe is on bedrest until the baby is born, but her intense food cravings lured her to Shawn and Gus' food truck. Shawn informs her that Lassiter is on his way down there. Marlowe panics because she knows it will upset Carlton to find her not at home on bedrest. She also knows he hates food trucks, so he'll be extra upset. Marlowe hides in the truck just as Henry shows up needing to talk to Shawn. Shawn then notices Lassiter's squad car pulling up, so he tells Henry to take care of Marlowe who is hiding in the food truck. Henry is utterly confused, but does so anyway. Inside the vegan food truck, Shawn notices evidence that links Vito to the bank robbery. Shawn now knows what actually happened. Mauricio found out that Vito robbed the bank, so Vito killed Mauricio to keep him quiet. Vito's crime partner Jake found this out and told him he didn't want anything to do with murder. Vito was worried Jake was going to tell the cops, so he killed him in order to keep him quiet.
Vito knows he's been caught, so he drives away in his food truck. Just as this is happening, Henry yells out that Marlowe's water just broke. In a panic, Lassiter calls in a police escort and drives the food truck straight to the hospital. Amidst all the action, Shawn is still trying to come to terms with the sale of his childhood home. But Henry is able to soften the blow by telling Shawn that he sold it to a family that will take good care of it: Marlowe, Lassiter and their baby. Henry coaches Marlowe through the delivery and just as they arrive at the hospital the baby comes out. Henry hands Lassiter and Marlowe their newborn baby girl and they are instantly smitten with her. Meanwhile, the police eventually catch up with Vito's truck and have him arrested. The case is solved and Marlowe and Lassiter are officially parents.