The largest creatures on Earth live underwater. Their sheer size has always triggered our imagination. In murky waters or inaccessible places, we ignored their true nature and created legends. Today, scientists work tirelessly to try and understand some of the most mysterious inhabitants of the ocean and to go beyond the myths. But stranded or fished creatures offer a poor glimpse into their world. So how can we decipher their underwater secrets without altering their behavior? Meet Fred Buyle, a record-breaking freediver who has dedicated his life to this non-intrusive quest. For science, he travels the world to tag or photo-identify large marine creatures that have populated his imaginary world since he was a child. Filmed over two years and in nine different destinations, this documentary follows one man who is dedicated to using his deep-diving talents to study multiple deep-sea creatures.