The Land Before Time All Grown Up XI: Mystery of the Monsterousaurus is a 2030 theatrical animated film and in The Land Before Time All Grown Up series featuring new cast members.
A herd of dinosaurs were eating green food and drinking water when a ferocious rogue longneck named Red Foot attacks and in the bushes green eyes of a howling dinosaur appears.
In the Great Valley, Littlefoot (Eljiah Wood) and his friends Ducky (Tara Strong), Petrie (Tom Kenny), Spike (Mitchell Whitefield), Tippy (Rickey D'Shon Collins), Chomper (James Arnold Taylor) and Ruby (Grey Delisle) at the new treesweet tree where they get ready to have some treesweets for the Fall Celebration and they know Cera (Hayden Panettiere) is gonna be the first one to get one and she teases Littlefoot for being so little the last time and Littlefoot gets upset and leaves.
Meanwhile Mr. Thicknose gets a visit from his old friend Terra (Queen Latifah) and Cera notice him talking to someone.
Meanwhile Littlefoot tries to reach the cliff but couldn't reach it and then at his home, he tried to get the tree stars from his tree and then mess it up and pasted out and then he had a nightmare about a longneck that looks like him with red and yellow eyes and sharp teeth and woke up after he screamed. Later, a gray two-horn (Nasutoceratops) named Nasuto (Kevin Michael Richardson) and his boys have the tree run out of treesweets cause a monsterous longneck who attacked many dinosaurs in every valley and heads off to the Forest Valley where some young horned dinosaurs are and Cera takes the others to go there and they want Littlefoot to come with them.
Later, Littlefoot and his friends head off the Forest Valley and Mr. Thicknose tries to cheer up Littlefoot for saying something about something that makes him upset for being so little.
As they head for the valley Littlefoot tries to make paths but somethings are too big for him so Cera can do her own things.
When Littlefoot and his friends arrived at the Forest Valley, they meet Mr. Kosmos the dinosaur with lots of horns on his frilled (Kosmosceratops) who is the leader of the valley and they know who Cera is cause they heard about her father Mr. Threehorns.
Back at The Cave Chomper, Ruby, and the others heard what Horin was saying when they know that he and his siblings and friends were using them to get all the attention from Mr. Kosmos and everyone so they got mad at them and quit and they're no longer friends with Cera who is the reason why Littlefoot ran away and walked out on them.
When Cera , Horin and the others tried to find Chomper and the others they ran into Mr.Kosmos and Mr.Kosmos was so mad at Horin for using his guests after what Hornberto said and no choice but to put him and his siblings and friends but Cera on punishment forever and told them to march themselves right home and told them not to leave there and not to patrol ever.
At Dawn, Mr. Kosmo told everyone that Horin and his siblings and friends but Cera are all on punishment forever for using Cera's friends and leaving Littlefoot out.
At a river Chomper and the gang are outcast from their home cause they'll get lost if they can't find their way back without Littlefoot and encountered to Longneck Swimmers (Elasmosaurus) who are really mad that Ducky took their eggs by mistake.
Meanwhile, when Cera found Littlefoot, she wants him to come back to her father's home to stop Red Foot, but Littlefoot refuse cause she might use him again, and yells at her telling her to get lost and he never wants to see her again, and Cera got sad and ran away from him.
During the sky water, Cera sadly walk back to her father's home after being yelled by Littlefoot, feeling abandon by her own friends.
After the sky water stopped, Cera saw Mr. Kosmo who was heading to a meeting and told him that she lied to him and said that Littlefoot found Red Foot first before she did after chasing his name to Tall Tail and she missed the old Littlefoot and Mr. Kosmo was glad that she told him and he told her in the valley they don't make fun of dinosaurs. And Tandy who heard what Cera said finds out that Tall Tail is Littlefoot.
Meanwhile, Littlefoot who yelled Cera, is still very angry that he can't reach things that bigger and higher and fell down into a pit, and in the forest, Red Foot who smell Littlefoot's sense open a path that's leads to the Mysterious Beyond with his tail and let in a pack of Fast Biters (Dromeosaurus).
Later, Chomper and the others were about to find Littlefoot but looking for his tracks but then encountered the fast biters that got in and attack them and they fight back with the help from the Tiny Longnecks and just when the fast biters were about to get them they smelled something coming their way and runs back to the path they came in and it was Red Foot and Red Foot was looking for Littlefoot and then attack Chomper and the Gang.
Back in the Forest Valley, Tandy tells Trayson and the others that Tall Tail is really Littlefoot when he change his name and Tad overheard her what she was saying and Centro warns them about Chomper and the others are fighting Red Foot but Tad say that is Cera's business, and Tandy and the other along went out to help them.
Back in the forest outside the Valley, Spike and Tippy got wounded by Red Foot so was Petrie and Ducky and Ruby who got scratched after weaking him, and Chomper who bravely stands up for his friends and when Tandy and everyone found them they were too late and sees Chomper fighting Red Foot and weaked him by biting and scratching him but Red Foot scratched him in the mouth and hit him with his tail and then discovered tracks that came from Littlefoot and set off to find him. Tad thinks he's looking for Cera but Mr. Nammer told him Red Foot not cause it something to do with those tracks that came from Littlefoot.
Back in the valley, Cera heard that her friends were injured by Red Foot and was on his way to kill Littlefoot like Big Daddy said Red Foot hunts those he hasn't before and gets Horin and the others and when she got back to them she told them that she lied to them about finding Red Foot after teasing Littlefoot and goes to save Littlefoot.
At the pit Littlefoot tries his best to get out but keeps coming down and gets really upset for being to little and he meets Brighterra and then tries to climb out and was about to slide down but, grabbed the branch with his teeth and continues to climb until he was able to get to the top and then saw Red Foot's who found him and starts roaring so loud allowing Cera and the others know that he found Littlefoot.
After taking out Cera, Red Foot goes for Littlefoot so he can kill him but notice that he's vanished and then Littlefoot attack from behind and the fight between Littlefoot and Red Foot goes on when Littlefoot goes for Red Foot's foot and breaks it with his tail so Red Foot can't scratch him and was able to beat him by using his tail to knock him off the cliff into the crevasse and Red Foot was gone forever.
Elijah Wood as Littlefoot
Hayden Panettiere as Cera
Tara Strong as Ducky
Tom Kenny as Petrie
Mitchell Whitfield as Spike
Rickey D'Shon Collins as Tippy
James Arnold Taylor as Chomper
Grey Delisle as Ruby
Justin Long as Horin
Bruno Mars as Hornberto
Orlando Brown as Styraco (Styracosaurus)
as Chasmi (Chasmosaurus)
as Einio (Einiosaurus)
as Pyana ( Protoceratops)
as Rif (Protoceratops)
Idris Elba as Pento (Pentoceratops)
Nate Torrence as Centro (Centroceratops)
l Music Dinosaur Like Me by Phil Collins (Music of Littlefoot and his friends heading for the Forest Valley).
It's a Valley Out There by Philip Lawrence
There's No Way Back by Phil Collins (Music of Littlefoot quitting and giving up and running away when he didn't keep up with the others after a fight with Red Foot)
They are not Helping No more (Music of Horin and his siblings and friends but Cera who are on punishment for using Chomper and the others and leaving Littlefoot left out)
Friends No More by Trisha Yearwood(Music of Cera being sad after being yelled by Littlefoot who doesn't like threehorns anymore).