Plot Summary

Two years after the events of the second film, Dave (Jason Lee), the Chipmunks (Justin Long, Matthew Grey Gubler, and Jesse McCartney) and the Chipettes (Christina Applegate, Anna Ferris, and Amy Pohler) go on a cruise ship heading for the International Music Awards. Both groups end up creating trouble; culminating in Dave having dinner with the captain to apologize for the trouble. He tells them to stay in their room, only for all of them (except Theodore) to escape to the casinos. Dave discovers his former supervisor; Ian Hawke (David Cross) is working as the ship's safety monitor dressed as a pelican, and is out to inform the captain if the Chipmunks and Chipettes cause more problems. The next day, Alvin decides to go para-sailing on a kite but the kite flies away with him and the other Chipmunks. Dave goes on a hang-glider to try to find them but Ian attempts to stop him, which results in them both ending up in the Pacific Ocean.Meanwhile, the Chipmunks find an island and they sleep for the night. Dave enlists Ian's help to find the same island and begin looking for the Chipmunks. The next morning the Chipmunks go and find food and while doing so, an island castaway named Zoe (Jenny Slate) shows up and sees the chipmunks for the first time. They then go to Zoe's tree house where Eleanor sprains her ankle and Simon gets bitten by a spider; its side effects including personality changes and loss of inhibition.The morning after, everyone observes Simon's personality changed where he thinks he's a French adventurous chipmunk named "Simone." (Alan Tudyk) "Simone" becomes attracted to Jeanette but does not take as kindly to Alvin and Brittany. Later, Zoe takes "Simone", Jeanette, Eleanor and Theodore to a lake with a waterfall and "Simone" finds a cave. He returns with a gold bracelet which he gives to Jeanette as a crown. Brittany and Alvin see an active volcano the next day and they decide that they have to leave the island with the others. Theodore and "Simone" find Dave and Ian and they go to meet with the other chipmunks. They all begin to prepare a raft to get them off the island and everyone is assigned a job. When Jeanette and "Simone" go and look for food, "Simone" is knocked unconscious and Jeanette is kidnapped; "Simone" reverts back to Simon afterwards.Everyone finds Simon awake and he cannot remember anything since the bite. They discover that Zoe has taken Jeanette and they head towards the waterfall. When they approached the tree log to cross, Dave and Alvin decide to go and find Jeanette. As Zoe forces Jeanette to get the treasure in the cave by tying her to a rope, she reveals that she was never a castaway, but came to the island intentionally to find the treasure, but due to the effects of living alone on the island for ten years, she has gone mentally insane and is ruthlessly willing to find the treasure at all costs. Alvin and Dave come to her rescue. The island begins to rumble again and Zoe lets go of the rope and Jeanette runs with Dave and Alvin back to the raft. When they reach the log to cross, Dave almost falls.Alvin and a reformed Ian convince Zoe to help save Dave. They then run towards the raft and escape the eruption. While on the raft, Zoe apologizes to Jeanette for kidnapping her and forcing her to get the treasure. As a gift, Jeanette gives Zoe the gold bracelet that Simon had given to her. Alvin reconciles with Dave and they are rescued. The Chipmunks and Chipettes perform at the International Music Awards. Ian also starts a new career as a screenwriter by selling a screenplay about Zoe's story to Hollywood, earning him his wealth back and making Zoe famous. And the flight home Alvin talks into the microphone. And Dave yells "Allllvvviiiinnnnnnn!"