The film opens with Lara Croft (Angelina Jolie) in an Egyptian tomb, seeking a diamond at one end of a chamber. As she approaches she is attacked by a large robot. After an intense chase and battle, she disables it by ripping out its motivational circuits. She takes the diamond, which is revealed to be a memory card labeled 'Lara's Party Mix,' and inserts it into a laptop computer inside the robot, whereupon it plays music. Now it is revealed that the scene took place in a practice arena in Croft's home, and that her assistant Bryce (Noah Taylor) programmed the robot, SIMON, to challenge her in combat.



It is the day of the first phase of a planetary alignment, culminating in a solar eclipse on the Earth, which happens once every 5,000 years. In Venice, the Illuminati search for a key to rejoin halves of "the triangle," which must be done by the final phase of the alignment. Mr. Powell (Iain Glen), an Illuminati member, makes assurances that they are almost ready, but in reality he has no idea where to find the key.



Croft's butler Hillary (Chris Barrie) tries to interest her in several projects, but she ignores them. May 15, as Hillary is aware, is the day that Croft's father disappeared many years earlier. She has not recovered from his loss.



Later that night, Croft has a dream reminding her what her father said about the alignment, and an object linked to it called the Triangle of Light. Waking, she is aware of a clock ticking. Searching for it, she discovers a secret chamber with a carriage clock that had spontaneously begun ticking. Bryce probes it and discovers a strange device hidden inside the clock.



Since the device resembles a clock, Croft consults a clock expert friend of her father's, Mr. Wilson (Leslie Phillips). She believes it is connected to the "Triangle of Light," but Wilson disavows knowledge of the clock or the Triangle. Croft encounters Alex West (Daniel Craig), a fellow tomb raider with unscrupulous methods. They are attracted to each other, but Croft cannot abide his for-profit attitude. That night, Croft is contacted by Wilson, who tells her that he gave her name to a man named Manfred Powell in regards of the clock. In reality, Wilson is also a member of the Illuminati.



The next day, Croft sees Powell in his home, and shows him photographs of the clock. Later, while discussing it with Bryce, she points out that Powell was lying about his knowledge. That night, armed commandos invade the house and steal the clock despite Croft's attempts to fend them off.