Pi Patel (Irrfan Khan), an immigrant from Pondicherry in India living in Montreal, Canada, is approached by a local novelist (Rafe Spall) who has been referred to him by his "uncle" (a family friend), believing that Pi's life story would make a great book. Pi relates an extended tale:
His parents had named him Piscine Molitor after a swimming pool in France. He changes his name to "Pi" (the mathematical symbol,) when he begins secondary school (Gautam Belur), even repeating numerous digits of pi, because he is tired of being taunted with the nickname "Pissing Patel". His family owns a local zoo, and Pi takes an interest in the animals, especially a Bengal tiger named Richard Parker because of a clerical error. Pi tries to feed the tiger, endangering himself to being attacked, and to teach him the reality of the tiger's nature as a carnivore, Pi's father, Santosh Patel (Adil Hussain) forces him to witness it killing a goat. He is raised Hindu and vegetarian, but at 12 years old, he is introduced to Christianity and then Islam, and starts to follow all three religions as he "just wants to love God." When asked if he is also Jewish, he replies that he lectures in Kabbalah at the university.
When Pi is 16 (Ayush Tandon), his father decides to close the zoo and move his family to Canada, and sell the zoo animals, to ensure a good future for his children. They book passage with their animals (to be sold in North America) on a Japanese freighter named the Tsimtsum. On board the ship, Pi's father gets into an argument with the ship's cook (Gerard Depardieu) when he speaks to Pi's mother, Gita Patel (Tabu) rudely. One night, the ship encounters a heavy storm and begins to sink while Pi is on deck marveling at the storm. He tries to find his family, but a crew member throws him into a lifeboat; from the rough sea, he watches helplessly as the ship sinks, killing his family and its crew.
After the storm, Pi finds himself in the lifeboat with an injured zebra, and is joined by an orangutan, named Orange Juice, who lost her offspring in the shipwreck. A spotted hyena emerges from the tarp covering half of the boat, and kills the zebra. To Pi's distress, the hyena also mortally wounds the orangutan in a fight. Suddenly Richard Parker emerges from under the tarp, and kills and eats the hyena.
Pi finds emergency food and water rations on the boat. He builds a small raft of flotation devices so that he can stay at a safe distance from the tiger. Realizing that he must feed the tiger to protect himself, Pi begins fishing, with some success. He also collects rain water for both to drink. At one point, he makes a board ladder for the tiger to climb back into the boat after it had jumped off to hunt fish. In a nighttime encounter with a breaching whale, Pi loses much of his supplies. Faced with starvation, he eats raw fish. After many days at sea, Pi realizes that he can no longer live on the tiny raft and trains the tiger Richard Parker to accept him in the boat. He also realizes that caring for the tiger is keeping him alive.
After weeks longer at sea, near the end of their strength, they reach a floating island of edible plants, supporting a forest, fresh water pools, and a large population of meerkats. Both Pi and Richard Parker eat and drink freely and regain strength. But at night the island transforms into a hostile environment, with the fresh water turning acidic, digesting all the dead fish that died in the pools. The tiger returns to the lifeboat at night. Pi finds a human tooth inside a plant flower and concludes that the plants are carnivorous, requiring them to leave the island.
The lifeboat eventually reaches the coast of Mexico. Finally back on land, Richard Parker stumbles away from Pi and stops at the edge of the jungle. Pi expects that the tiger will turn toward him and acknowledge him, but instead he looks into the jungle for a while and goes in. Pi, too weak to follow, lies in the sand. He is rescued by a group who carry him to hospital, but he weeps that the tiger had walked away without him.
In hospital, insurance agents for the Japanese freighter come to hear his account of the incident. They find his story unbelievable, and ask him to tell them what "really" happened, if only for the credibility of their report. He answers with a less fantastic but detailed account of sharing the lifeboat with his mother, a sailor with a broken leg, and the cook. In this story, the cook kills the sailor to use him as bait and food. In a later struggle, Pi's mother pushes him to safety on a smaller raft, and the cook stabs her as she falls overboard to the sharks. Later, Pi returns to grab the knife and kills the cook.
In the present, the writer notes parallels between the two stories: the orangutan was Pi's mother, the zebra was the sailor, the hyena was the cook, and Richard Parker, the tiger, was Pi himself. Pi asks him which story the writer prefers; he chooses the one with the tiger because it "is the better story", to which Pi responds, "And so it is with God". Glancing at a copy of the insurance report, the writer notices a closing comment about the remarkable feat of surviving 227 days at sea, especially with a tiger - meaning that the agents chose that story as well.