Sam may not get along with her wealthy, snobby parents, but being their daughter does have its advantages, like having your own greenhouse, where Danny and Tucker find her that morning, watering her plants and picking her lunch. To his friends' confusion, Danny's shivering despite the heat inside and the parka he's wearing — no matter what he does, he just can't get warm today. Ah, It's Probably Nothing, just a bug he caught or something. The three leave for school without noticing the half-ghost leave behind a trail of ice that swirls around a plant, freezing it solid.
They pass by a construction site on their way through town, prompting Sam to get on her soapbox about the dangers of messing with nature. Cue a bunch of plants splitting the asphalt as they pop up out of the ground before forming into a giant ghost made of vines: Undergrowth, hater of all human activity that encroaches on land that rightfully belongs to plants — read: the whole planet. He immediately goes into action, tearing up the construction site, vowing to make humans pay for the pain they've caused his kind.
Even Sam finds his approach a little too extreme. Danny goes ghost and gets to work. Unfortunately, he soon learns he's dealing with one of those annoying enemies who can instantly heal and regenerate — no matter what part of him Danny blasts or destroys, it just grows back. Danny charges up a huge energy blast, doubting he can regenerate after being completely reduced to ashes, only to be stopped short by another shivering attack. Undergrowth binds him and his friends in vines and douses them with some sort of giant pollen that Danny quickly realizes is sleep spores.
By the time Danny wakes up, the entire town has been covered in giant, thorny vines and flowers, tearing up buildings, streets, and sidewalks everywhere they grow. He and Tucker are bound high above the ground to a giant tree by vines that he can't break or phase through. Undergrowth reappears to monologue about how "Mankind is merely a temporary weed in the garden of life — a weed that can be removed!" and show them how he plans on doing that: he's using his "mind vines" to take control of all the humans and set them to work destroying the city for him, tilling the soil, and planting seeds so his "children" can grow and spread and Take Over the World.
Still shivering, Danny tries to brag that even if he can't stop him, somebody else will. His parents show up on cue in the Fenton Ghost Assault Vehicle but only get to battle the plants for about one minute before they're taken over by the mind vines. While Danny yells at Undergrowth to release his parents, Tucker looks around at all the people he now has under his control and realizes the two of them must be the only ones left. Danny, however, notices someone else who's missing: "Where's Sam?"
"Ah, yes, the female — every garden needs a caretaker..." One of Undergrowth's giant flowers rises and opens to release Sam, wrapped in his mind vines, and wearing a spiked collar, spiked garter, and a green, leafy, Stripperific dress that makes Poison Ivy's wardrobe look modest. Turns out Undergrowth was impressed enough by her "love of vegetation" to make her his Dragon. While the boys are still reeling from the sight, their possessed friend snags Tucker with one of the mind vines and prepares to have a giant, fanged flower-monster devour Danny.
Fortunately, this triggers the Die or Fly response, and Danny — shivering even more and glowing blue — spontaneously freezes all the plants holding him, turning them to solid ice. He's able to break free and fly away, but Sam and her plants go after him. More vines grab his arms, pulling him into his second Crucified Hero Shot of the episode, before they also freeze and break. The only way he can think of to escape is to fly higher: "No such thing as flying plants!" True, but there is such a thing as plants that can grow tall enough that that's not a problem, as Undergrowth quickly reminds him: "I am EVERYWHERE!" Danny blasts at the vines surrounding him, creating a small opening big enough for him to slip through.
He starts shivering again as he flies, and ice now starts forming around him, making him so cold that he can't stay airborne. He turns intangible as he plummets to the ground, passing through the street into the sewer, where Sam is waiting for him. He doesn't bother trying to fly again but just runs for it, eventually climbing out of a manhole that brings him up right in front of his house. Covered in ice again, he runs inside as the army of vines continues to chase after him. He's so frozen he can barely move at this point, but he manages to phase down into the basement. The vines grab him again as he heads for the portal into the Ghost Zone, but they freeze as soon as they touch him, and he breaks away easily. Sam tries to stop him, asking him to stay and rule with her; whether this is a disinhibited Sam saying what she really wants, or Undergrowth making her say what he thinks Danny would want to hear, it doesn't work. Vowing to come back and save her, Danny turns intangible and runs through her into the portal.
Still freezing but out of reach of the plants, Danny drifts through the Ghost Zone. He's conscious but weak and hopes he can steer himself towards the one person who can help him. He eventually reaches a snowy, frozen landscape, where he crashes into the ground and passes out. He's found almost immediately by two Yeti-like ghosts, one of whom recognizes him as a friend and says they need to take him back to the city.
"Good morning, Sleeping Beauty!" (Er, wrong Disney princess...) Danny wakes up this time floating in a de-icing chamber (and stripped to his underwear) in a med-lab in the Realm of the Far Frozen. He's a bit unnerved by his current state but relieved to see his old friend Frostbite. He explains how Undergrowth has taken over his hometown and how he tried to fight him but was crippled by the "cold sensation" overtaking him. Frostbite's heard of Undergrowth, and he's familiar with Danny's ice power; it's a power many ghosts have, including his people, who are "quite well-trained in its use" (as he demonstrates), and he sensed it in Danny the first time they met. Danny eagerly asks him to teach him.
Danny's shivering uncontrollably again when he meets Frostbite for training in an outdoor amphitheater, where dozens of his people have gathered to watch. Frostbite explains that Danny's problem with his ice power is that "ever since you acquired your ghost abilities, you have only been letting it out in small amounts" — to control it, "you must learn to let it out all at once." Danny closes his eyes, takes a deep breath, and lets it go, releasing a strong burst of cold in a storm of blue and white energy. He inadvertently freezes his mentor solid, but it works; he's not shivering anymore. A short Training Montage follows in which many innocent bystanders get frozen, but Frosbite doesn't mind, except for the effect it will have on snack sales.
Danny thanks Frostbite and his people for their help as he prepares to leave, although he still doubts that he'll be able to defeat Undergrowth. He's afraid to return home and wonders if it would be better if he just stayed here forever. Frostbite tells him he can't live in fear: "Fear is natural, Danny Phantom — charging into battle despite the fear is what makes someone a hero. A hero must always believe that he will win, and your new abilities just may help." Somewhat reassured and fully resolved to do what he has to do, Danny bids them all good-bye and takes off to face whatever's waiting for him back on Earth.
His basement is dark and almost completely covered in thorny vines when Danny comes through the portal. Prepared for the worst when he sees the rest of the town, he phases up and outside. Sure enough, the vines have covered everything so that the whole city looks like a green wasteland, with not a single human or other living creature in sight. Danny hears Sam calling his name and flies toward the source, but everywhere he sees is completely deserted. And in this kingdom of isolation, it looks like Sam's the queen. Now dressed like Maleficent, she's glad to see that Danny has returned, even though she's otherwise quite happy with her new home. Danny asks what she's done with everyone, and she shows him a bunch of people (including his family and Tucker) sleeping in giant pods; now that they're work is done, they'll be fed to her "children."
Undergrowth now joins them, and the Final Battle begins. Still nervous about using his ice power, Danny at first fights back using only his energy blasts. He puts up a good fight but finally gets curb stomped in a Battle of the Still Frames before getting tied from head to toe in a bunch of vines. He tries to tell Sam to stop, but this Mind Control apparently isn't as easy to resist as Freakshow's. Undaunted, she has her vines gag him and drag him underground to finish him off and feed him to the plants.
Underground, Danny struggles fruitlessly against the vines, remembering how Frostbite taught him how to concentrate and release his energy safely. Realizing It's the Only Way, Danny closes his eyes, concentrates, and screams, "Let! Me! GO!" as the pit of vines surrounding him completely freezes over. With a blast of icy energy, he rises out of the ground, glowing icy blue like he's in the Avatar State. He shoots an icy blast at Undergrowth, encasing him all over in a frozen shell. Undergrowth, however, doesn't freeze and break like his offshoots but shakes it off and bursts free, diving at Danny for another strike. The hero dodges the attack and freezes him again, longer this time, but he still breaks out of it.
Undergrowth finally wraps Danny in more of his vines, but Danny turns on the ice again, freezing them. This time, he notices Undergrowth shrink as soon as the vines break. Now knowing what he needs to do to kill this weed, Danny turns intangible and dives underground, freezing all of the roots one by one with blasts of ice. As his roots freeze and die, Undergrowth shrinks more and more, and even the plants above ground freeze over. Undergrowth rants about how he can't do this until Danny rises up to deliver the Finishing Move. He generates an icy blast in his hand, but instead of releasing it, he generates an ectoplasmic energy blast inside it. He then hurls the white and green swirling ball of energy at Undergrowth. The energy blast breaks through the vines that make up his body, which, of course, instantly grow back... but the ice now begins to freeze him from within. There's no way he can break out this time, not when the ice is inside him; he glows bright blue and screams ''NOOO!" before exploding in a burst of ice and blue light.
All the ice now instantly melts as all the plants rapidly wither, turn black, and die. The mind vines all come loose, freeing everyone from their control. Everyone who was trapped in the pods wakes up on the ground, disoriented but unharmed. Sam wakes up in her normal clothes and her eyes their normal color, with a grassy taste in her mouth and no memory of what happened. Tucker stands up next to her right before Danny walks toward them, still surrounded by an icy blue aura, confusing them even more. He doesn't take the time to explain yet, but he does show them his new powers by making a small ice crystal just like he saw Frostbite make. Sam's impressed, but she's not sure how to react when Danny gives it to her. They share a brief smile and Headbutt of Love as he tells her he's glad she's okay before Tucker kills the moment by riding in on a lawn mower, deciding now is a highly-opportune time to open such a business. He starts by mowing down the tiny version of Undergrowth, already beginning to regenerate, that pops up by Sam's feet, causing everyone to laugh in time for the episode to end before the implications of that can sink in.
This episode provides examples of:
- Actor Allusion: You can't have Mark Hamill in your cast and not give him a Big "NO!", can you?
- A Friend in Need: Frostbite says this when he finds Danny passed out in the snow.
- An Ice Person: Add this to Danny's Combo Platter Powers.
- Be Careful What You Wish For: The Far Frozeners watched Danny's ice training hoping he'd comically mess up. He does, but unfortunately, they end up on the receiving end of it.
- Big "NO!":
- When Danny, worried he might not be able to save his town, asks if maybe he could stay in the Far Frozen, the Frozeners he had injured while training shout this.
- Undergrowth lets a truly epic one when Danny beats him.
- Brainwashed and Crazy: Both Danny and Tucker have been subjected to this already; it's only fair that Sam now gets her turn.
- The Cat Came Back: Even after Danny completely destroys him, Undergrowth still regenerates, albeit small enough for Tucker to shred with a lawn mower.
- Chekhov's Gun: Danny's chilling ghost sense that he's had since Day 1 has always been an extension of the ice powers he's never used until now.
- Chekhov's Gunman: Frostbite and the rest of his people in the Realm of the Far Frozen whom Danny met in "Infinite Realms" (which hadn't aired yet...).
- Come to Gawk:Danny: Why are there so many of your people watching us?Frostbite: It is not every moon that my people get an opportunity to see their leader train one as legendary as you, O Great One.Danny: You mean they're hoping I'll mess up, huh?Frostbite: Indeed. These people live in a frozen wasteland — they take their comedy where they can get it.
- Creepy Monotone: While brainwashed by Undergrowth, Sam speaks in a much more calmer tone. It's quite creepy.
- Either/Or Title: The episode was advertised as "Phantom Freeze".
- Elemental Eye Colors: Danny Phantom's normally ghostly green eyes glow icy blue when he uses his ice powers.
- Evil Costume Switch: Sam gets two; the first one also invokes...
- Go-Go Enslavement:Danny: Sam...Tucker: (flustered) Green's a good... color on you...
- Gondor Calls for Aid: Why Danny flees to the Ghost Zone.Danny: Only one person who can help me...
- Heroic Vow:Danny: (to the possessed Sam) I'll be back... and I'll save you. And everyone! I promise!
- Hope Spot: Danny experiences one when his parents show up to fight Undergrowth. It's very short-lived.
- How Do I Shot Web?: At least Danny has a competent if eccentric mentor to help him master his newest ability.
- "Join Us" Drone: Sam becomes a puppet of Undergrowth and tries to get Danny to join her in serving him. Naturally Danny isn't interested.
- Know When to Fold Them: Seeing that the first battle is going poorly as well as his ice powers hindering him. Danny makes a break for the Ghost Zone as it's the only place Undergrowth has no control over. He just barely gets there before a hypnotized Sam can stop him.
- Hypnotize the Princess: Undergrowth hypnotizes Sam into being his Dragon.
- Man of Kryptonite: What Danny ultimately becomes to Undergrowth once he master his ice powers. His regular powers could only hinder Undergrowth briefly due to his regenerative powers. But ice is harmful to plants, so once Danny starts using them directly, he manages to legitimately start harming Undergrowth until he depowers him and ultimately blows him up.
- Meaningful Echo: "Cold air is bad for the plants!"
- Mind-Control Eyes: The eyes of everyone under control of the mind vines turn solid green, including Sam.
- No Ontological Inertia: Destroying Undergrowth destroys all of the plants and frees everyone from his Mind Control.
- Point That Somewhere Else: After Danny accidentally skewers some Far Frozeners with ice spikes, they run away when he waves goodbye to them.
- Power Incontinence: (Un)Fortunately, Danny's ice powers freeze him instead of the area around him before he learns to control them.
- Sealed Evil in a Can: Averted for the first and only time in the series — Undergrowth blows up at the end of the Final Battle, making him the only ghost to be (seemingly) destroyed instead of trapped in the Fenton Thermos and sent back to the Ghost Zone.
- Sequel Hook: Subverted. Though seemingly defeated by Danny, Undergrowth later returns as a shrub...but Tucker ends up running him over with his lawnmower.
- Shout-Out: Two of Sam's outfits while under Undergrowth control are ones to Batman (Her first resembling Poison Ivy) and the second resembling Maleficent from Sleeping Beauty.
- Survival Mantra: "I control the power — the power doesn't control me."
- We Can Rule Together:Sam: Stay, Danny... Stay and rule with me!
- Whole Plot Reference: Call it "A Ghost Grows In Amity Park".