NOTE: This spoiler was submitted by Jeremy

The film opens in a house where a tennis ball falls off the shelf and begins rolling around the house. Inside is Marcel the Shell (Jenny Slate), a tiny shell with one eye and shoes. Documentarian Dean Fleischer-Camp (playing himself) films Marcel’s day-to-day life as he lives with his grandmother Connie (Isabella Rossellini), and a piece of lint tied to a string that Marcel calls Alan.

Marcel explains to Dean how he gets around the house throughout the day. Dean tells Marcel that he is renting out the AirBnB as he is looking for a place to stay since he just got divorced. Marcel shows Dean the various mechanisms he uses to get around, and how he takes in the beauty of the outside world. He and Connie enjoy watching “60 Minutes” and are fans of Lesley Stahl. However, Marcel is sometimes sad remembering that he used to watch the show with his and Connie’s whole family.

Marcel shows Dean carvings of his family that he put on a table. He had a mother and father, siblings, aunts, and even neighbors. Early on, the shells lived among a couple, Mark (Thomas Mann) and Larissa (Rosa Salazar). They would argue a lot, and the shells would come together. After they broke up, Mark took his bags that Marcel’s family got caught in, and he has not seem them since then. He spends his days with Connie, but she appears to show signs of dementia.

Dean uploads Marcel’s videos to the internet, and they become surprisingly popular due to Marcel’s cuteness and insight on the world around him. TV personalities such as Conan O’Brien and Brian Williams do coverage on Marcel, and he earns many fans in this new phenomenon. Dean helps him set up a livestream video where he asks for help in relocating his family. When he posts his home address, he only receives a bunch of people standing outside just wanting selfies with the house. This leaves Marcel more disappointed than pleased.

Dean offers to take Marcel out around in his car as they try to locate Mark’s car. Marcel gets carsick and is also overwhelmed when he sees just how much bigger the town is compared to his own backyard when Dean takes him to the top of a hill. They end up going home without anymore leads. When they get home, Marcel finds that Connie fell from the washing machine and cracked her shell. He decides to stick around and take care of her more.

Dean finds a voicemail from “60 Minutes” producer Shari Finklestein, stating that the show would like to do an interview with Marcel and Dean. Marcel is hesitant, as he is worrying about Connie’s health. He says he won’t do it until she feels better. Dean tells this to Connie, who later tells Marcel to go through with it despite his concerns because she wants him to live a more fulfilled life, and also tells him it may bring him closer back to his family.

The crew arrives at the house for the interview. While Marcel is nervous at first, Dean helps him come out to meet everyone, including Lesley Stahl, leaving him and Connie starstruck. Marcel and Dean go through with the interview, but after it’s done, they cannot find Connie. Marcel goes to Connie’s bed and finds that she passed away. Dean helps Marcel bury Connie in her garden. Marcel delivers a eulogy for his grandmother and mourns her. Meanwhile, Dean signs a lease on a new apartment.

Marcel receives a follow-up message from “60 Minutes”, as their segment on him led to them tracking Larissa down in Guatemala. Despite Mark’s reluctance, Larissa gets the crew to bring Marcel and Dean to Mark’s house to find Marcel’s family. Mark and Larissa begin to argue, which reminds Marcel that whenever the shells heard the two fighting, they would huddle together for security. This leads to Marcel and Dean finding the family in Mark’s drawers. Marcel happily reunites with them as they huddle around him.

Dean helps Marcel bring the family back to the house, where they settle back in. Marcel sings them a song as they have a proper funeral for Connie, and the family talks to the camera while enjoying their time together.

The last scene shows Marcel in the laundry room as he looks out the window. He tells Dean how he likes the sound that comes out when he stands by the window and the wind blows through his shell while he admires the beauty outside.

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Marcel the Shell has a documentarian, Dean, observe him and his grandmother Connie. Marcel's cuteness and insight into the world around him leads to him becoming a major phenomenon. He uses the newfound popularity to help track down his family, who got taken when one of the previous owners of the house moved away.

Connie's health begins to deteriorate, leading Marcel to worry about taking care of her. When "60 Minutes" wants to do an interview with Marcel, he is hesitant because of Connie, but she and Dean encourage him to go through with it. During the segment, Connie passes away.

Marcel mourns Connie but is later told that the "60 Minutes" crew tracked down the ex-girlfriend of the guy who took Marcel's family. She brings the crew, plus Marcel and Dean, to the ex's house, where they indeed locate Marcel's family and bring them back home with him.