Role in the film
Matai Shang is the Hekkador of the Therns, his goals are mysterious and enigmatic. He and the Therns are a race that travel the universe, organizing and feeding off the destruction of planets, manipulating civilizations in order to do so. It is his and the Therns' purpose to do so, however, as Shang mentions at one point that he and the Therns have been doing it for billions of years.
In the beginning of the film, the Therns arrive on Barsoom (Mars), and witness two cities, Helium and Zodanga, at war with each other, and decide to equip Zodanga with a mysterious weapon called the 9th Ray. Matai Shang and the Therns ally themselves with Sab Than, the Jeddak of Zodanga, who plans to marry the Princess of Helium, Dejah Thoris, and kill her during the wedding ceremony. When that happens, Sab Than will rule Barsoom forever, causing it to end life there, and nature will follow its intended course, which is what Matai Shang and the Therns want.
Matai Shang and the Therns help Sab Than ensure his plan goes smoothly using various means including sending a Thern to destroy Dejah's invention just as she was about to show Helium the Ninth Ray. Later, Sab Than and Shang come up with an idea to make Dejah Sab Than's wife and allow them to complete their plan of Sab Than becoming ruler. In order to do this, Shang manipulates the Great Martian Clan of Warhoon to pursue John Carter an earthling who mysteriously arrived on Mars, Dejah and their Thark companion, Sola, and the Martian beast Woola, allowing Sab Than to rescue Dejah and make her reluctantly agree to marry Sab Than to end the war.
As the wedding is planned, Matai Shang meets John Carter and they go on a walk throughout Zodanga. During their walk, Shang explains to Carter the purpose of the Therns and how they travel the universe and manipulate civilizations of different planets. He also mentions that the Ninth Ray weapon must remain in the hands of Sab Than, while Dejah, and anyone else with knowledge of the Ninth Ray, will die. He then reveals to Carter Sab Than's plan to rule Barsoom, and that when Sab Than kills Dejah, her death will seal Sab Than's reign forever, completing the course the Therns have set for Barsoom. Woola, Carter's companion, suddenly appears and attacks Shang, allowing Carter to make his escape on a flying ship. Shang, unbothered by this, returns to Helium to watch Sab Than's wedding.
However, his and Sab Than's plans are forever foiled by John Carter, resulting in the death of Sab Than and the end of the war. During the battle, Shang confronts Carter and tells him that Dejah will live, but on the condition that she will fail to prove her theory of the Ninth Ray. He then drops his medallion and attempts to gain it back from Carter, Dejah and Tars Tarkas. While he is successful at retrieving his medallion, he is forced to leave the battle as Helium wins the war. Not too long after Carter's victory, Shang, using the medallion, sends Carter back to Earth before he himself leaves Mars forever.
After spending 10 years on Earth looking for any evidence of the Therns' existence on Earth and hoping to find one of their medallions, Carter is apparently able to find a Thern medallion and seemingly returned to Barsoom, leaving his body in a coma-like state on Earth inside a large mausoleum that can be only opened from the inside. Having made his nephew, Edgar Rice Burroughs, his protector and gave him clues how to open the tomb to protect his body from the Therns, Edgar, whom Carter calls "Ned", races to the mausoleum to open it, hoping to find Carter's body. At that same moment, Shang, who had been observing all these events on Earth in the form of a man with a bowler hat, walks behind an unknowing Ned and prepares to destroy Carter's body, but both he and Ned are taken aback to see the mausoleum is empty. Suddenly, a gunshot rings out and Shang drops dead. The shooter is revealed to be Carter, who had not found a medallion and instead laid a clever trap for Shang in order to lure him out of hiding and gain his medallion, to finally return back to Mars.
- Matai Shang states that he as a Thern can live forever, but John Carter counters that Therns can be killed, as proven by him killing one Thern at the beginning and ultimately killing Shang at the end, meaning that the Therns may not have always been immortal, and that it's likely due to their advanced technology that Matai Shang and the other Therns live as long as they do.