In Indiana Jones and the Last Crusade, Indiana’s father uses the Grail diary to identify the Holy Grail.
Henry Jones Sr. has spent years researching the Grail, recording clues, sketches, historical references, and observations in a personal diary. When Indiana reaches the temple, the diary helps him recognize the correct cup among many vessels. The key clue is that the Grail belonged to a humble carpenter, so it should be a simple cup rather than an ornate royal goblet.
The diary is central to the film’s father-and-son story. Henry initially treats the search as an academic mission, while Indiana becomes the person who must apply the diary’s information under pressure. Their shared knowledge ultimately helps them survive the temple’s tests.
The diary is not the Grail itself. It is a research record, and its importance is sometimes confused with the artifact’s final resting place or with the cup chosen during the climax.