The episode begins with a title card saying "Previously" and the narrator speaks, and several clips of past episodes were shown. Then we come back to the scene where Gumball, Richard, Nicole, and Darwin were trying to take a picture, and Darwin accidentally getting flushed down the toilet.
The episode then begins with Darwin coming out of the sewer and into the sea, meeting several other fishes. He frantically asks them how to get home. The other fish said a fish can't get far on land, but Darwin never gives up. He swims faster and jumps out of the sea, but collapses quickly on the sand due to him not having lungs.
Back at home, we see Gumball with a tear in his eye at the loss of his new friend. Nicole and Richard try to cheer him up, and Gumball says that he believes Darwin will return, going back to the scene where Darwin gets his new heart and lungs. He waves goodbye to the fish and begins his journey back home to Elmore. Across the deserts and the valleys, Darwin gets tired, but soon sees that he is close to Elmore. He runs happily, but bumps into a rock.
Back at home, Gumball is still sad, but he wished he'd kick Darwin in the butt if he gave up his journey, and we go back to Darwin, who has now grown new legs and feet. Darwin tries out his new legs and feet, and runs all the way to Elmore.
Darwin, now happy that he is in Elmore, asks several citizens where his pal Gumball is at, only to scare them. While Nicole, Richard, and Gumball were posting missing papers about Darwin, a Pest Controller shoots Darwin and locks him up in a cage where the dogs were. But after he sensed that Gumball gave up, he took the key to unlock the cages of the dogs and himself, freeing himself and the dogs. Darwin then happily continues to find his family.
The Wattersons decide to get a new "Darwin," not noticing Darwin running past behind them while they drove away in their car. At the Elmore Mall, Darwin finally catches up to his family but notices that the Wattersons are about to replace him, leaving him heartbroken and sadly leaves the mall. However, just as Gumball is about to purchase a new fish, he senses Darwin's presence and realizes he's back. He runs to catch up with Darwin and they reunite outside the mall, overjoyed. Much to Richard and Nicole's joy. Noticing Darwin's bruised feet, Gumball gives him his green shoes. The Wattersons officially adopt Darwin as a member of their family and drive back home as Darwin prepares to explain how he managed to return to Elmore, ending the episode.
It is revealed that Sussie was quite intelligent and speaks in full sentences as an infant, but after her parents accidentally dropped her, her voice and intelligence changed as she became the way she is now.
[The episode starts of with the subtitles "PREVIOUSLY," then the narrator begins speaking.]
Narrator: [speaking very fast] Previously on The Amazing World of Gumball...
[Then a bunch of scenes from previous episodes pop up instantaneously and briefly before cutting to where the previous episode had left off.]
Narrator: [speaking very fast] But more importantly, the Wattersons have adopted a magical fish called Darwin who they love very much but...accidentally flushed down the toilet.
[Darwin then jumps out of his fishbowl, and lands on the top of the bathroom toilet, on the flusher, and into the toilet, and gets flushed away. The other three watch with shock on their faces as they watch Darwin get flushed farther down the toilet. The view is also in Darwin's perspective.]
[We then see a fading effect, as the scenes then cuts to Darwin being blasted out a sewage exit in the sea, where he then finds a swarm of catfish, near the exit.]
Darwin: Ahhh! What happened? Where am I? Who am I? How do I get home? What is life? Okay okay, calm down. I got flushed, I'm in the sea, my name's Darwin, I don't know how to get home, and life is what you make of it! Everything is cool! Ahhh... [Relaxes for a moment before starting the same process again] No wait! I'm lost! I need help! How do I get back to Elmore? [Stops for a moment until he realizes what's wrong] Oh right. You guys only speak fish. [gurgling, but is seen moving his tongue up and down] How do I get back to Elmore?
The Catfish: Ohhh.
[They all go to the top of the sea]
Catfish: Woah woah woah, let me get this straight. Your mom's a cat, your dad's a rabbit, and they made a fish?
Darwin: No. They bought me from a magic van.
[The fish flap their fins in laughter]
Catfish: And they flushed you down what?
Darwin: A toilet. It's a thing they sit on to... you know. [Fish flap their fins in laughter] And I ended up here cause all the waste gets flushed on to the sea. [The Catfish glare at Darwin] Anyway, thanks for showing me the way back to land.
Catfish: Yeah but fish can't survive on land.
Darwin: I'll find a way. Love always finds a way.
Catfish: You said a lot of crazy things today, but that is by far the most stupid! [Fish flap their fins] You can't survive out of water!
Darwin: [determined] I have no other choice.
[Darwin then swims down, and we see Darwin still swimming down, but this time with a more realistic scene of the ocean waves as he continuously swims farther down underwater to attempt to make a landing.]
[Scene cuts back to Darwin, who is still swimming down underwater with all of his might. Then he jumps out of the water in slow-motion. The scene the cuts to where the catfish are, now all looking at Darwin.]
Catfish: He's gonna make it!
[All the catfish are still looking at Darwin, who is still seen jumping out of the water in slow-motion, then after about a second, he falls onto the nearby shore, face flat on the sand, not moving at all.]
Catfish: Eh, maybe not.
[Camera zooms into Darwin's heart and Transitions into Gumball's heart]
Nicole: I know your sad sweetie, but maybe it's time to- [Tries to take away Darwin's fishbowl from Gumball's hands]
Gumball: No! He'll need this when he comes back.
Gumball: I know he's trying to get back. I can feel it, here. [Points to his chest]
Richard: In your stomach?
Nicole: In his heart Richard. Not everyone's stomach fills their whole chest.
Gumball: Come back buddy... I believe in you.
[Camera then zooms into Gumball's heart which then transitions to Darwin's heart which starts beating and grows lungs. Darwin then takes a deep gasp of air as he wakes up. The fish rejoice.]
Catfish: Told you he'd make it.
Darwin: Nice to meet you fish friends, but now I must begin my credible journey, to hurry back to my family. Goodbye! [Starts crawling away from the beach. He encounters a pair of flippers and gasps. He looks up to see that no one is wearing them. He sighs with relief and keeps going]
He's on His Way!
[As he sings, various sceneries are shown as he crawls through them]
Darwin: I'm on my way! I'm on my way! I've got my lungs, and that's a start, and I've got love inside my heart. I'm on [underwater] my way! [Back on land] I'm on my way! I may be tiny and alone, but I won't stop until I'm home. I'm on my way! I'm on my... [his stomach growls] way! And though you hear my stomach rumble, [he eats what appears to be a worm only to find it's an opposum's tail] I'll still make it back to Gumball. [The rat hisses and Darwin backs off. Time lapse. He's tired] Across the deserts and the valleys, past the bears that growl and roar. At last I've made it home to Elmore! [He's on a map] Aw… [Slower] Still on my way! Still on my way! And though the journey's pretty slow, I know I'm gonna find my bro. I've reached the top. And I won't--[A bird catches him and keeps him in its mouth to take him back to its baby, but Darwin escapes from its mouth, slaps the baby and crawls away] ... stop. My heart will find with... Gumball. I won't stop, give up, or stumble. [sees a shadow in the distance resembling Gumball] Gumball! [Darwin rushes to it only to find out it is a pile of rocks that only look like Gumball] Okay, that was half my bad, half- [Rock drops on his head] ... rock. Gumball... I don't think I can make it. I'll just lie down here for a while and... [Camera starts fading on Darwin's quite tired face]
[Transition into Watersons' house where Richard painted his mouth to resemble Darwin and stayed behind the fishbowl]
Richard: Look Gumball, we've tried everything we can to make you feel better. But I don't think Darwin is ever coming back.
Gumball: No. I'm not going to give up! Darwin wouldn't give up, and if he did I'd be there for him and I'd... I'd kick him in the butt.
[We then see the warped-like tunnel again, which travels back to Darwin's heart, and his heart can be seen again, just as it could be seen previously.]
Gumball: [voice echoing] Kick him in the butt...
[After Gumball's voice can be heard, Darwin's heart suddenly grows a pair of legs to literally represent Gumball's sentence of encouragement.]
Darwin: [Darwin's butt gets kicked] Ow! [He get's kicked again] Huh? Ow! [He gets kicked a third time and camera zooms out to reveal Darwin having legs] What? I've got legs. Ahhh! What is happening to me? Ahhh! I can't run away from them! Wait a minute, I can't run at all. :[Stands upright and sees a city in the distance] Elmore! Huh? It looks pretty small. Oh it's not small, it's really far away. How do people use these things? There must be an easier way! All fours! That's better. [Transition into Elmore] Yes! I made it! Do you know where The Wattersons live? They have a son called Gumball, and he's my best friend. Do you know? He looks like this! [Transforms his face to look like Gumball]
Crocodile Woman: Ahhh! [Scared at him, she drops her potato chips]
Darwin: Thank you! I'm really hungry! [eats the chips]
[He then runs along to the park, where a young Teri can be seen riding on a little bunny springer.]
Darwin: Hello! I'm looking for my friend Gumball!
[Teri then floats away along with the wind off screen]
Darwin: Is she okay? [The swing she was riding on rocks back and forth and Darwin thinks its nodding] Oh good!
Sussie: When I grow up, I want to be a lawyer like you father.
Sussie's Parents: Ahhh! A talking dog! [Sussie's parents accidentally drop her]
[Sussie makes weird noises]
Darwin: Hey! Hey! Hello! Hello!
Jackie: Somebody call pest control!
[Transition into Wattersons' family again to find them all outside trying to put posters up to help find Darwin]
Nicole: [Reading the poster Richard made] Have you seen this boy's fish?
Gumball: Just put up a picture of Darwin!
Richard: Oh. Sorry. [Puts up a picture of Darwin] There.
Gumball: No no no, not just Darwin! [Sighs] Buddy... please come back.
[Both their hearts beat at the same time]
Darwin: Gumball! Gumbal- [Gets shot with a tranquilizing dart by the Pest Controller and is taken to his van in a cage but then wakes up when the van passes the Watterson house and both him and Gumball's heart beat at the same time, they also both gasp] Gumball! Let me out! I need to get back to my family!
Sausage Dog: Family huh? I used to have one of those too. You need to let go son. The sooner you forget about them, the less it'll hurt.
Darwin: But they were right there. I... I need to get back to them. I need to get back... I need to get back...
[The Wattersons' house]
Gumball: [Looks at one of Darwin's posters and rips one out of the post only to see that there are many more beneath it] What the? Dad!
Richard: What? You said to put up all the posters.
Gumball: Not in the same place! [Wind blows all the posters away as Gumball reaches to get them, but fails, and falls to his knees crying] It's no good. We'll never find him anyway.
[Camera zooms into his heart which turns black and hard and blows up]
Gumball: [Voice echoing] We'll never find him anyway..
Darwin: [Feels his heart blowing up and knows he gave up, he gasps] He gave up... It's all over. This is the end. There's no way out.
Sausage Dog: Oh the keys!
Darwin: I know, I know you were right. The key to this is to just let go.
Sausage Dog: No no no no, you open the-
Darwin: I opened my heart once and look what happened!
Sausage Dog: No you take the keys and you open-
Show Dog: THE KEY! YOU'RE STANDING RIGHT NEXT TO THE KEY!
Darwin: [Grabs the keys] Come on guys! Let's get out of here! Goodbye! Good luck finding your families!
Nicole: Come on honey.
Richard: Cheer up son! You know what they say! There's plenty more-
Nicole: You were gonna say... you know.
Richard: I was gonna say there's plenty more love in a heart for a new friend.
Nicole: People don't say that! They say that there's plenty fish in the sea- [Covers mouth]
Richard: [Shakes head] Too soon.
Gumball: It's okay. I guess it's time to get a new Darwin...
[They get into the car and drive away and we see Darwin running towards them in the distance]
Darwin: I'm here! Wait! No! I am here! Don't replace me! I'm here! Waiiiiit!
Nicole: Hold on! the bowl is like fifteen times the price of the fish.
[The Wattersons drive off as Darwin runs after them]
Darwin: [Waves arms around]
Nicole: Look! The Rainbow Factory's opening. Maybe I could get a job there.
Darwin: What? No!
Richard: You won't need a job once my buisness idea pays off.
Nicole: What buisness idea?
Darwin: You! Follow that car!
Darwin: No! With me! With me!
Richard: It's called the internet. It's a place where you exchange different types of nets. Fishing nets, hair nets, butterfly nets..... So you just hire different nets with different sizes!
Larry: Sir, I have my own net.
Darwin: I'm here! I'm here! Don't replace me!
Larry: Here you go little fella.
[Darwin sees Gumball getting a new fish and sadly walks out the parking lot but as he walks away Gumball's heart starts beating again]
Gumball: [whispers] Darwin...
Gumball: No. I can't! There's only one Darwin! [He walks out of the store and closes his eyes and concentrates and his heart beats again] He's here! Darwin's here! I can feel it!
Nicole: Gumball, wait!
[At the parking lot Gumball and Darwin finally see each other]
[Gumball and Darwin run in slow-motion towards each other and hug]
Nicole: You came back!
Gumball: You have legs? How?
Darwin: I don't know how, but I know why.
Richard: [Gasps] It wasn't for revenge, was it?
Darwin: It's okay. I know you didn't flush me on purpose, Mr. Watterson.
Richard: Please son. Call me dad.
Darwin: Okay, Mr. Dad.
Nicole: Darwin! Your little feet.
Gumball: Wait! Wait! Hold on... here. [Gives Darwin his shoes]
Darwin: Thank you. I'll never take them off.
Nicole: So...how did you manage to make it back to Elmore?
Darwin: I'll tell you in the form of a song.
[A song is about to begin with the bird, the fish and the dogs singing a tune but Gumball and Darwin are already asleep. Richard then shushes Nicole so they can sleep and they drive home as the sun sets.]