Christopher Nolans Interstellar (2014) features one of science fiction most mind-bending endings. Understanding what happens requires grasping several complex scientific concepts.
The Tesseract
In the film climax, Cooper ejects from his ship and finds himself inside a strange construct - the Tesseract. This is a three-dimensional representation of time, created by future humans to allow Cooper to send data to the past.
How the Tesseract Works
Inside the Tesseract: time becomes a physical dimension Cooper can navigate, he can see Murph room at different points in time, he can affect gravity in specific moments, and the bookshelf connects past and present through gravity.
The Message
Cooper discovers that his future self is the "ghost" Murph heard in her childhood. He manipulates the watch hands to send binary code - the quantum data that allows scientists to solve gravity and save humanity.
The Brand Resolution
Professor Brand spent his life on "Plan A" (save Earth) but knew it required data he did not have. He lied to keep Cooper motivated. Brand died not knowing if her data would work. At the end, Cooper retrieves Brand from Edmunds planet and she reveals she lived among survivors who successfully evacuated Earth.
Time Dilation
The film accurately depicts gravitational time dilation: Water planet has 1 hour equaling 7 years near the black hole. Cooper returns and 23 years have passed for Romilly. This is based on Einstein general relativity - time moves slower near massive objects.
The Ending
Cooper escapes the Tesseract and is rescued near Saturn. He reunites with elderly Murph who tells him to go find Brand. The film ends with Cooper piloting toward Brand colony. "Do not go gentle into that good night" - Cooper chooses to go, not accepting death but seeking new adventures.