The Sentinel is a science fiction novel written by Arthur C. Clarke. The story follows a man named Richard who is assigned to a remote outpost on the moon. His only companion is an advanced computer system named Hal. As Richard spends more time at the outpost, he begins to notice strange occurrences and starts to question his own sanity.
One day, Richard discovers that the outpost is actually a monitoring station for an alien race that has been observing Earth for centuries. The aliens have been waiting for humanity to reach a certain level of technological advancement before making contact. Richard must now decide whether to reveal this information to the world or keep it a secret.
The Sentinel explores themes of isolation, discovery, and the consequences of contact with extraterrestrial beings. It is a thought-provoking and suspenseful tale that will keep readers on the edge of their seats until the very end.