Groot is a superhero.
Little is known about the history of Groot. He is a plant-like creature that looks like wood.
At some point, when he was just a small sapling-like creature, he was captured by Collector. He was put in a cell and stored in a room along with a butterfly, Adam Warlock's cocoon, a praying mantis, two jellyfish, birds, plants, and various other creatures. It seems that Collector saw Groot as a highly prized part of his collection taking extra care of him.
Groot was depressed at his imprisonment and loneliness.
After some time, Rocket Raccoon appeared next to his cell and began laughing. Groot spotted him and became excited at the thought of freedom and waved.
However, Collector appeared on the other side of the cell and scowled at the rescuer. Rocked looked shocked and ran away. Collector ran around the cell to follow. Groot could only watch them go.
Rocket ran down amongst the other cells while Collector followed. Eventually they ran out of sight.
After some time, Rocket ran back to Groot alone. He examined the cell and Groot bounced over to him. Somehow, Rocket got Groot out and carried him away leaving a doll imitation of Groot behind.
Rocket stopped nearby and the two watched the cell.
Collector ran by the cell and stopped. He pulled the Groot doll out and hugged it. Soon the doll fell over and the eyes fell off. Collector threw the doll onto the ground smashing the pot. He then leapt up and down in a tantrum before kicking the doll.
Rocket and Groot laughed as he did. Rocket held his fist up and Groot bumped it.
It is unknown what happened afterwards. Presumably the two soon escaped Collector's lair.
Groot was unnamed and unvoiced. He made a few noises but it is unknown who made them.
The concept of Groot's capture is based on a scene in the Guardians of the Galaxy film. In it, Collector asks if he can have Groot's remains after he dies. While Groot apparently dies, he survives in a smaller form. This short follows if Collector had taken that smaller form as they had discussed earlier.
The design of Groot is based on the fan favorite "Baby" Groot seen at the end of the film.