In his studio in Sterling, Virginia, Derrick repairs the wear and tear on active dog Samson's foot prosthetic. In Lexington, North Carolina, six-month-old miniature pony Lightning needs braces to compensate for the weak ligaments that prevent him from standing pain-free.

Derrick 3-D prints a leg prosthetic for Waddles the Duck. Avery, a golden retriever in Port Orange, Florida, needs partial-limb prosthetics for her back paws which were damaged when she was born.

In Plainfield, Indiana, Derrick fits Chihuahua puppies Turbo-Roo and Thor, who have no front legs, with 3D-printed carts.

In the studio, Derrick builds Jack Russell mix Callie a prosthetic for a missing foot. And a goat named Ferdie in Urbana, Ohio needs a prosthetic for an amputated leg.

In the studio, Derrick 3-d prints a jacket for a full-limb prosthetic for Lt. Dan, a rescue dog who lost his leg from buckshot. But his triangle stance causes issues. In Erie, Pennsylvania, Bert the pig suffers from a connective tissue disorder and requires prosthetics on all four legs.

In New York City Pearl, a French Bulldog rescued from a Chinese slaughterhouse, is missing a front paw. Her owner in addition to giving her PT wants to get her a prosthetic. Derrick tries out a new design for her prosthetic and switches a caster for a crutch tip when she proves reluctant to use it. In the studio, Laci, a black lab needs a prosthetic, but Derrick's worried if there's enough elbow left for good attachment and suspension.

In the studio, Rocky the golden retriever has a missing front paw. In Orlando, Florida, five-year-old dog Elsa is missing all four of her paws. Her pet parents have been building booties for her daily with bandages and tape. But her stumps are still tender. And Derrick knows getting an animal to accept four devices at once is a longshot.

In the studio, 10 year old golden retriever Zoe whose leg was recently amputated for a tumor needs a brace to keeper her good leg from collapsing at the wrist, and a whole-limb prosthetic for the missing leg. In Boston, Derrick builds a partial limb prosthetics for Gracie, a dog missing a back paw, but she's rejected a prosthetic before.

In York, Pennsylvania, at the "Here With Us" farm sanctuary, Derrick helps Bear the goat who's missing the bottom half of both his back legs and is handstand walking on his front elbows. He needs six devices: rear leg prosthetics, and bent knee braces to force his rear end down at first, then straight knee prosthetics to eventually straighten the front legs and get him up on all fours and reteach him to walk. So Derrick asks his twin brother Darrell to be an extra pair of hands. For Ziggy's Refuge Farm Sanctuary, Derrick builds prosthetics for 3-week old pig Hanna Montana, with deformed rear legs, and St. Croix sheep Moko whose back legs were lost to frostbite needs new updated braces and prosthetics after surgery.

In a two-part episode, Derrick travels to Paro, Bhutan to visit Baryard Bhutan sanctuary to help them build their own prosthetics workshop. He also builds a prosthetic for Yusi the horse who's nursing a colt but the missing portion of her rear leg is causing her spine to twist out of alignment, but the first design doesn't work.

Derrick introduces his family and describes how the business has exploded and he's taking the mobile prosthetics unit (and Henry) cross-country. In Lancaster, Pennsylvania he meets with truck driver Elvin to help his 10-year-old canine campanion, Toto who lost her front paws in a farm accident two months ago. Toto's still sensitive at her incisions. Derrick creates a clam-shell design to redistribute the weight off the ends of her legs, and decorates her pink prosthetics with butterflies to match the braces Elvin's granddaughter uses. Derrick and the crew also get in some inflatable knocker action.

At High Point, North Carolina, Derrick video calls his family from the world's largest set of dresser drawers. Michael and Linda, dwarf pony Lightning's owners, have asked Derrick for help with their matriarch dog Lacey, whose back end is paralyzed from degenerative disk disease injuring her spine. Derrick sets out to build her a cart, but the full body cast is larger than anything he's done in the mobile lab before. Derrick also adjusts the fit on the four braces that Lightning's brother, Thunder, needs.

In Benson, Vermont, Derrick visits the Kinder Way Farm Sanctuary to help Aries, an Icelandic sheep who spends all day on his belly. He has contracted tendons in his front legs, weakness in his back limbs, and has Windswept Syndrome (leans to one side) from a birth defect, so he can't balance in a standard cart. With all four limbs non-functional. Derrick designs his first quad cart. They also ask Derrick to look at Lois, a cow who has contracted tendons in both front legs. He's confident he can build devices that will immediately relieve her pain and help straighten her legs out over time to fix the issue. But Aries has an infection and dies just before the fitting; the family donates the cart componentry. Lois accepts her braces and learns to walk in them.

In Los Angeles, Derrick sightsees in Hollywood before going to help Greg and Jamie with Shadow, an 8-month-old lab/pitbull mix born with front leg deformities who's unable to stand or walk and bounces off her chest/chin to get around. Jamie was also born with a limb difference of the right arm and feels fellowship with Shadow. Derrick expects to build a front limb cart, but he'll have to fit it around her residual limbs. Derrick explains a full body cast is the hardest to make and keep the animal comfortable, but Shadow's completely relaxed. He designs it to use off-road wheels with enough axle width for stability and enough space for residual limb movement. But this makes him mount the componentry farther back than he usually would and he has to adjust angles to get the right height. They go with butterflies for the printed pattern. Jamie wore a passive prosthetic that was painful throughout her childhood but she freed herself from it after graduating high school. But she feels Shadow's prosthetic making a functional difference to improve her quality of life is worth doing despite the love/hate relationship she had with her own. Derrick uses the parts from Aries' cart. Shadow is hesitant, but after Derrick moves her from dirt to pavement, she can walk!

In Huntington Beach, California,^[4] Derrick meets friends Tara and Lisa and their dog, Frida. On a trip to work at an elephant sanctuary in Thailand, they saw Frida dragging herself through traffic and immediately rescued her. She was missing flesh and bones from her feet and had an infection, and a broken and dislocated back. The lower part of her legs are paralyzed and they now bend backwards; she's walking on her rear knees. She's only comfortable on the soft sand at the beach. Derrick tries to surf. In the consultation, it becomes clear Frida doesn't trust Derrick. He decides to cast the back legs twice and build devices in both her current position to protect her joints and make more surfaces comfortable to walk on as well with the joints in their correct positions. Frida takes swim therapy. Derrick fits the comfortable braces first and she takes off running and playing with Henry. He thinks it would be a miracle if the ideal position braces work, but tries them anyway. And Frida quickly takes to them and walks in normal posture. Derrick is amazed. And now Frida trusts Derrick.

In Alliance, Ohio, Derrick fails an ice cream eating challenge at Almost Heaven before heading to Canton, Ohio to treat Zoey. Zoey is an 8-month old golden retriever who lost the front right paw in an accident at birth. Owner Cindy previously had a three-legged golden retriever named Wrigley and she knows how his life was shortened and his pain increased by not having a prosthesis. And having had a leg amputated, herself, from osteosarcoma at 14, Cindy's well aware of the benefits—and difficulties—of using one. Derek geeks out over Cindy's leg before designing Zoey a prosthesis that's adjustable in height and angle since she's still a puppy and not fully grown. Cindy is also an amputee mentor and wants Zoey to be a therapy dog to help her support new amputees adjusting to a first prosthesis. Derrick uses the same componentry for Zoey's device that Cindy's has. Zoey panics and doesn't take to the prosthetic immediately, but she gradually begins to touch/tap, and Cindy knows it's a process that takes time. She's also glad that the story of Zoey having to work and learn to use the leg will be a good one for her mentees to hear.

Trigger is a pig in Providence, North Carolina with disabled back legs.

At Magical Creatures Sanctuary, an animal sanctuary in Laupāhoehoe, Hawaii, the young sheep Menina was born with deformities in her front legs. Derrick attempts to build prosthetic legs for her.

In a three-part season finale, Derrick travels to Thailand to fit elephants Mai Mai and Khun Dej and a bull named Mr. Cow with the braces and prosthetics he's been built for them.

In Raleigh, North Carolina, Derrick helps Kaitlyn surprise her sister Kamryn by creating a full-limb prosthetic for Kamryn’s dog Amiri after he lost his leg in a golf cart accident.

Derrick travels to California to meet Martin and Jesse Kove, stars of the “Cobra Kai” franchise, and help create a full-limb prosthetic for their 3-legged dog, Bubba.

At an animal sanctuary in Renton, Washington, Derrick builds a brace for Amelia, a cow with birth defects. He adds height to her shortened back leg to fix her triangle stance and hopefully extend her life.

In Ontario, Canada, Derrick meets best friends Winnie, a two-year-old 'kangaroo' dog with front limb deformities, and Chip, a six-month-old sheep. Derrick builds them both custom carts. Chips takes to the cart immediately, but Winnie...does not.

In Deltona, Florida, Derrick brings hope to a mother and daughter as he crafts a custom cart for their potbelly pig named Valentine. In the studio, Derrick builds pitbull cancer survivor, Emily Rose, a full-limb prosthetic.

The Safe in Austin rescue in Texas needs help lifting their quadriplegic cow, Champion, off the ground without making him uncomfortable. Unable to make a whole body cast of such a large animal, Derrick must call on the expertise of friends to help him build a new and better-fitting lift to fly a cow.

At Nolin River Wildlife Sanctuary, Derrick builds a cart for Morrison, Boone's buddy, another raccoon with cerebellar hypoplasia. In Louisville, Kentucky, Derrick builds front-feet prostheses for pitbull Hanabi who lost her feet at birth, with a little help from her human brothers who want to lose a footrace to her.

In San Francisco, Derrick helps out a family with a one year old German Shepherd whose left rear foot was severed. Her residual limb is still tender and sensitive and makes walking painful. Derrick hopes he can make the prosthetic comfortable enough that she won't reject it.

In the studio, Derek builds a prosthetic for a five-month old Flemish Giant rabbit, Diego, whose broken foot healed without medical care into a permanently curved limb. In Wayne, Illinois, Derrick has to build a full-limb prosthetic for a 10-year old tripod dog, Birdie. But since she had to have the leg amputated shortly after birth, she's never been on four legs before.

Derrick sets up a pop-up in LA and begins interviewing potential clients.

Derrick struggles to finish the mammoth task of making 13 devices in three days, with only his mobile lab.

In Monticello, Florida, Derrick visits the Juniper Wildlife Rescue, run by Zach and Jess, to help a fox named Fig. Fig was born with a missing foot but used the shortened leg like a crutch. Unfortunately, a break in the leg required partial amputation and now it's too short to be used as a support, and remaining ankle is collapsing. Fig is Derrick's first fox, so he must study the other foxes at the sanctuary to gauge the correct angles for the brace he'll build to help the remaining leg. When fitted, Fig panics at first, but quickly adapts to the brace.

In Elkton, Maryland, Derrick stops at an apple butter festival before going to help the Sotnik family's Corgi, Ollie, born with split-paw ectrodactyly. The shorter legs and lack of a pronounced elbow to anchor a prosthetic on his first Corgi patient worry Derrick. He gifts the family with six hydrotherapy sessions for Ollie, something they couldn't afford. Ollie adjusts well and even runs in his new prosthetic.

In Fort Collins, Colorado, at Broken Shovels Farm Sanctuary, Agatha is a year and a half old goat with malformed back legs who handstands vertically on her front legs. Derrick needs to design a unique (tie-dye) cart to support her, but also allows her to adjust angle to what's comfortable. The first build's straps need to be redone before Agatha accepts the cart.

Derrick travels to Montrose, Colorado to help Ivy, a miniature cow with front limb deformities.