Joining the Overwatch Team

After obtaining her MD and PHD in Switzerland, Ziegler rose to become the head of surgery at a prominent Swiss hospital before pioneering a breakthrough in the field of applied nanobiology that radically improved the treatment of life-threatening illnesses and injuries. It was this expertise that attracted the attention of Overwatch. She personally met with Commander Jack Morrison at Overwatch HQ. He was impressed with a paper she'd written on nanobiotic healing. She believed that it had the potential to completely revolutionize the way that medical care was given, not just in the examination room, but also throughout the entire structure of the medical profession. She believed that Overwatch would give her the opportunity to quickly get her ideas out into the field. Morrison told her that he wanted to become head of Overwatch's medical research. Ziegler was greatly tempted by the offer, but stated her reservations. She wanted to focus on the civilian and peacetime applications of her work. She didn't want to create new ways for Overwatch commanders to send soldiers into danger. Morrison was cordial in his response, and pointed out that if she joined Overwatch, she wouldn't have to worry about grants or haggle for new equipment. Whatever she needed, she'd get. Smiling, Ziegler agreed to join Overwatch.

Early Service

Because her parents had been taken by war, Ziegler was opposed to the organization's militaristic approach to keeping global peace. Ultimately, she recognized that Overwatch offered her the opportunity to save lives on a much larger scale. As Overwatch's head of medical research, Angela sought to leverage her work for healing in frontline crises. The result was the Valkyrie swift-response suit, which Ziegler herself piloted on many Overwatch missions. However, this was a point of contention for her. The Valkyrie suit had been created to prove her technology worked. But her teammates wanted her to stand shoulder-to-shoulder with them, and she acquiesed. This marked the beginning of her "Mercy" persona, while Doctor Angela Ziegler faded into the background.

At some point she and an Overwatch team helped people in Venezuela in the aftermath of a massive storm.

Despite her contributions to Overwatch, she was often at odds with her superiors and the organization's overarching aims. One example was the development of the biotic rifle by Torbjörn. Torbjörn provided assurances that the device was only intended for healing, but Ziegler maintained that this was setting Overwatch down a road that could lead to the weaponization of biotic technology, and that she had only conducted research in the field on the condition that it would not be used for offensive purposes.

Retribution

Eight years before the present day, Ziegler treated Gerard Lacroix for injuries he received after surviving a Talon attack on the Blackwatch Rome facility. She reported that she expected him to make a full recovery.

Uprising

Seven years before the present day, Ziegler was involved in an unsanctioned mission authorized by Strike Commander Jack Morrison to end the King's Row Uprising in London, England. Earlier the same day, Ziegler argued with Morrison over Overwatch's inability to intervene in the conflict due to the English prime minister expressively forbidding of any Overwatch action within the United Kingdom. Dr. Ziegler expressed her concern for the thousands in the city who needed medical aid, and lamented how Overwatch was made to help people, not sit on the sidelines. After Jack authorized the mission, while preparing to board an aircraft, Ziegler welcomed Cadet Lena Oxton to the team. She commented on how her inspirational words convinced Jack to authorize the mission, and how he was very invested in her career, and had asked Ziegler to keep an eye on her. Ziegler then told Oxton how she could call on her if she ever needed any help.

Storm Rising

Six years before the present day, Ziegler took part in an operation in Havana alongside Winston, Tracer, and Genji. Under the command of Captain Sojourn, their mission was to extract Maximilien, an omnic in league with Talon. The Talon forces were moving through the city in a convoy; one which the Overwatch agents engaged. Mercy swept over the convoy, causing one of the vehicles to crash. Fighting their way through the city on foot, the team was able to apprehend Maximilien, who suggested they could make a deal. Mercy said that he was in no position to negotiate and that they already knew about his associates. However, when asked what Overwatch was in the market for, Mercy stated "an introduction." In the end a deal was indeed struck, as Maximilien told Overwatch that Doomfist would be in Singapore in three weeks and in exchange requested that Overwatch not tell Doomfist of his betrayal.

Overwatch's End

In Overwatch's final days, the group was faced with a series of allegations, and Ziegler was interrogated in the hearings conducted by the United Nations. In light of the fight between Overwatch members Jack Morrison and Gabriel Reyes, Ziegler admitted that the outcome had been inevitable. They had drifted further and further apart ever since Morrison had been chosen over Reyes for the position of strike commander, and that rift had widened despite her efforts.

Egypt

Ziegler worked in an aid camp on the outskirts of Cairo for two years. She held Overwatch responsible for much of the country's suffering, and wanted to help fix it. However, the people there told her to "Go home. You've done enough damage." Nevertheless, she remained in the country and worked as a doctor, reminding herself that she hadn't become a doctor to be thanked. She may have applied to work at a proper hospital, but as she commented, "being a prominent ex-Overwatch official isn’t the sort of experience on your CV that people are looking for."

Recall

When Winston issued a recall of Overwatch agents, Ziegler was in the Middle East, performing triage in a makeshift tent clinic. She was unsure if Overwatch should be restored. She reasoned that doing the same thing they'd done in the past would only lead to disaster. She didn't doubt Winston's heart was in the right place, but that didn't mean he was right.

Valkyrie

Ziegler was reunified with Jack Morrison and Ana Amari as they crashed in her appartment. Morrison had been wounded by Reaper, and said that all he needed was a few medkits. Ziegler couldn't help but feel some irritation with Ana, as she was using a biotic rifle; an adaptation of Ziegler's technology that had been made without her approval. While she was happy that both of them were alive, both of them were still a manifestation of a past she had sought to escape. They asked why she was in Egypt, and why she wasn't working at a proper hospital, but she avoided answering both questions. Nevertheless, Morrison stated that he was hunting "the truth" about what happened to Overwatch, and, seeing Ziegler's skepticism, asked if he should answer Winston's recall order. She told him that vengeance wouldn't achieve anything, but he scoffed at the assertion. Ziegler fell asleep, but was awoken by an explosion. Morrison and Ana told her that someone had attacked the Temple of Anubis, and that they had to go. Ziegler protested that Helix Security would handle it, but Morrison pointed out that the attackers were Talon. Ziegler acquiesed to the pair, and decided to join them, and donned her Valkyrie suit. She flew over Cairo and at the Temple, saw the firefight between Helix and Talon. Morrison and Ana went after Reaper, despite her protests that the people caught in the crossfire came first. Flying down into the smoke, she found a young girl named Hanan pinned under rubble. She healed the girl's injuries using her biotic staff and told her that she'd fly her to safety. However, Hanan said that they needed to save her brother as well, who was still trapped in the collapsed building. The pair descended into the structure and managed to find Hanan's brother. The three ran as the building collapsed around them. As the building collapsed, Ziegler used her body to shield Hanan's brother, losing conciousness.

Ziegler regained conciousness after a period of time, finding Morrison above her. Ana told her that she had Hanan, and they got out of the building. The following day was a flurry of activity, as Ziegler and Cairo's medical services treated the injured. By the day's end, she was exhausted, and was only still awake thanks to coffee. At the aid camp, Ana and Morrison came to see her. Morrison told her that Reaper was still alive, and that they intended to follow him to Europe. Ana asked Ziegler to come with them, but the doctor refused. Ana and Ziegler parted on good terms, accepting that they were following different paths in life. She reflected how she'd stood at their graves when they'd been presumed dead. This time, she doubted she'd see them again.

Awhile later, Ziegler discussed things with Mahmoud; a co-worker in the camp. He told her that Hanan's parents were dead, and that they were looking for the children's next of kin. Watching Hanan and her brother, Ziegler reflected how recent events had awoken a sense of heroism within her. Feelings that she'd doubted would ever return. Watching Hanan in one of the aid tents, she reflected that her battle wasn't over.

Zero Hour

When Null Sector attacked Paris, Ziegler was among a number of former Overwatch agents who converged on the city to battle the attacking omnics. Descending from the sky in her trademark fashion, she healed Mei, who'd previously been injured. Together, they were able to defeat the enemy. In the aftermath, Winston declared that Overwatch was indeed back.

A New War

Following the battle in Paris, the team traveled to Watchpoint: Gibraltar, which they used as their base of operations. As it turned out, Paris was just the beginning of Null Sector's war against the world, and they weren't the only enemy Overwatch had to deal with. In conversation with Ziegler, Winston declared that they'd fight through one city at a time. She provided support for him at Route 66, where they fought their way through Null Sector's forces.