Cal-141 was abducted and conscripted into the SPARTAN-II program by Dr. Catherine Halsey in 2517 when she was just six years old. Cal was then taken to the planet Reach, and trained by the AI Déjà and Chief Petty Officer Mendez, along with the rest of the Spartan children.
Cal-141 was sent on an operation, along with the four Orbital Drop Shock Troopers she commanded, to assassinate a Minor Prophet and gather intel of unknown ruins on a Covenant-held planet, Heian. The group consisted of Dutch, O'Brien, Cortez and Checkman. Only three remained after the drop, as Checkman was killed in the upper atmosphere of the planet insertion.
During the operation, Cal fought a Brute Chieftain that wielded a Gravity Hammer and appeared to have been winning the fight. O'Brien hung for dear life over a waterfall as they fought, and was rescued by Cal soon after.
While waiting for the Prophet to arrive, the Brute Chieftain that Cal had supposedly killed the day before was still alive, and ambushed Cal and the three ODSTs. Cal spotted the Chieftain, about to strike O'Brien, and quickly moved to get the ODST sniper out of harm's way. She was hit across the head and right shoulder by the Chieftain's Gravity Hammer, resulting in fatal injuries, despite her armor taking the blow.
After the Chieftain was shot in the head repeatedly by Dutch, the trio removed Cal's helmet to inspect her injuries. They were stunned that Cal was a female, as opposed to the squad believing she was a male, especially O'Brien and Cortez. Cal, realizing that with her badly damaged arm, as well as her blurring vision due to her head injuries, she would no longer be able to take the shot, ordered O'Brien to kill the Prophet, which he succeeds in doing.
After O'Brien killed the Prophet, the ODSTs tried to tend to Cal's wounds, but she knew that she was not going to make it back to the Destroyer. In her dying words, Cal asked Cortez to make a delivery to Dr. Halsey. This delivery consisted of an optical chip from Cal's helmet, which contained intelligence of the planet the mission was carried out on, as well as ruins that were neither human nor Covenant. Cortez tried to assure her that she would be able to give the chip to Halsey herself, but Cal replied that the two of them knew it was not going to happen as she felt her death already moments away. She held Cortez's hands a bit longer and eventually died before the three ODSTs.
Cal's death left a profound impact on the ODSTs with whom she fought with on the mission. They, like all ODSTs, had a bitter rivalry with the Spartans and considered them to be freaks, but Cal's efforts in the mission and her actions in saving O'Brien's life many times until the loss of her own had changed their thoughts and opinions on the Spartans. This change was shown well as the ODST team later returned to their ship, where both Cortez and Dutch angrily scolded a marine for mocking Cal's combat credentials, and O'Brien pondered on how he might be dead if it hadn't been for her.
Publicly, she was likely classified as MIA as opposed to KIA, as per ONI policy regarding the deaths of the supposedly-invincible SPARTAN-II super soldiers. It is possible that in the end, O'Brien, Dutch, and Cortez either buried Cal or took her with them to the pick up point as her body is not seen in the same place where she died, although the former is more likely due to the fact that a SPARTAN can weigh upwards of 1000 lbs. when fully armored.