- "Because I see... I see what mankind is doing to this planet. The earth is doomed." - Tess Mercer to Jor-El, Kandor
Having released Zod and his troops via the Orb, Tess was held prisoner for three weeks in the Luthor Mansion. During this time, she was savagely interrogated by the soldiers and after meeting their leader, General Zod, she discovered that the Kandorians did not know how they came to be on Earth and that they are powerless.
When suddenly the whole army left the mansion, Tess revealed that letting herself be caught was only part of her plan to discovered more about them.
Tess started her search of Zod and his troops, which led her to an abandoned laboratory with stolen technology from LuthorCorp. Tess realized that the kryptonians were trying to get their powers in any possible way and after Tess's team recovered the remains of the kryptonian experiment, they revealed to her that Kryptonian symbols had appeared across the globe since the day that Zod arrived on the mansion. Tess then saw the House of El symbol in a picture and someone lying on it.
During her search, Tess investigated the disappearance of twelve sailors on a Luthorcorp facility until was attacked by rabid zombies that infected her with a strange virus. When Clark and Lois visited her to ask what had happened she aggressively attacked them turned into a zombie and was placed in quarantine to be studied in detail, as the doctors believed she was the center of the infection.
After being cured, Tess tracked Oliver down to Mexico and had him brought back to a shareholder's meeting in Metropolis at the Ace of Clubs. She made a "deal" with Toyman at his prison cell by giving him Metallo's kryptonite heart and enlisted him to discover how it worked.
As Tess displayed her twin solar tower to her meeting of LuthorCorp executives, she was ambushed by Zod, who congratulated her on her accomplishment. When Zod asked her to help him find the "Blur", she denied any information about knowing who it was. However, she did ask him for equal partnership, but he told her that no human would ever stand with a Kandorian. Not trusting Tess, Zod sent a soldier to kill her but instead she killed the soldier and returned his medallion to Zod, letting him know she is a force to be reckoned with.
Finally, Tess and Zod formed an alliance, although her primary motivation was to prevent Zod from finding the "Blur." Tess kidnapped Jor-El and convinced him to tell Zod that he was the Blur so that Clark could continue saving the world and when Clark learned that Tess had imprisoned his father, he threatened her by grabbing her neck and forced her to tell him where Jor-El was. She was happy that Clark had finally revealed his powers to her but not the way she expected. When she showed the location of Jor-El, Clark realized that Tess let Jor-El be kidnapped so that she could discover where the Kandorian army was.
Intrigued by the mysterious disappearance of Lois weeks ago, Tess kidnapped her in order to examine her memories of the missing three weeks. Tess requested the use of Summerholt technology to enter Lois's mind and saw the Solar Tower completed, realizing Lois was in the future. Tess connected her mind to Lois's mind and after discovering that she will be killed by Chloe, Tess began to question her allegiance to Zod.
One night, Tess went to her office at the Daily Planet and was shocked to find a chess-set piece on her desk. Tess then met with Amanda Waller at the Checkmate headquarters, who addressed her as "Agent Mercer" and said they had a lot to catch up on.
With the solar tower near completion, Zod decided to terminate his relationship with Tess and threatened to kill her. Then Tess tried to enlist Chloe´s help and suggested that they work together to stop Zod but, when Chloe refused. A Gemstone-kryptonite-infected Clark wanted to get his revenge on Tess for killing Jor-El and Tess pleaded with him for her life, saying that she did not kill Jor-El and reminded him that she had not revealed his secret to anyone. Chloe saved Tess and later Clark learned that Tess was telling the truth.
Tess angrily confronted Oliver about the destruction of her Solar Tower and embezzlement that she discovered within the company. She sent an agent to follow Oliver and discovered that he and Chloe had an arsenal of weapons. However, before she was able to get to them, Oliver had hidden them.
Tess arranged a meeting with Zod at the Kent farmhouse, where she shot him to prove that he had powers. She used green Kryptonite to weaken him and after talking of the superiority that some have over others they end up kissing and in bed. Zod asked Tess how she had weakened him in the barn and she told him they both have their secrets.
After Tess was summoned for a Checkmate operation to capture Green Arrow and later blamed when Green Arrow escaped, Amanda Waller threatened to kill her if she did not sort out the problem. When Tess discovered that Oliver was actually Green Arrow and helped him and Clark to shut down the power at Castle so they can save "Watchtower", Tess plead with him for help but Oliver showed no sympathy for Tess, as his trust in her had ended. Tess then revealed that she is "going underground", to prevent an assassination attempt from Checkmate
Tess resurfaced to retrieve John Corben when he escaped, and was discovered by Chloe who wanted to discovered how Clark became infected with red kryptonite. Together, they used their resources to stop Clark and Zod new partnership sending Corben to the Fortress of Solitude and as a desperate attempt to win support, Tess tries to manipulate Zod and informed him of some of the Kandorians betraying him to serve Kal-El, saying that she was the only one he could trust.
Tess returned to LuthorCorp only to find Chloe in Oliver´s office, hacking into the Swiss National Bank. She reminded Chloe that although she might be underground, she still ran Luthorcorp. The pair discussed Tess's alliance with the Kandorian army and Chloe commented on Tess's partnership with Zod, telling Tess that she'll run out of people to betray quickly and warned her about her decisions.
Tess came to the conclusion that Chloe was Watchtower and went after her, demanding all the information she possessed about the Kandorians. However, the two become trapped inside the building after the security system was activated by a Checkmate's protocol to access the system and both had to work together to try to escape alive. Tess revealed she has a GPS strip in her flesh that allows them to hack the computers and that burrows deeper into its host when threatened with extraction. In the midst of a touching conversation, Tess opens herself to Chloe and advises her about her relationship with Oliver. After making their way to get out of Watchtower, the two headed to Metropolis General Hospital knowing that Checkmate was following them to try to shut off the GPS strip. Chloe tells her the only way was killing her, and Tess accepted and put her trust in Chloe, knowing that she would let her die. After deactivating the chip, Chloe resurrected Tess, much to her surprise, and Tess then agreed to keep Chloe's secret as a form of compensation.
While being held hostage by Maxwell Lord using his telepathic abilities, Tess was accused of being the Red Queen and was required to disclose the location of the Book of Rao. However Tess escaped and went to find the Book of Rao she stole and hid in the wine cellar. Tess was later approached by the Red Queen, who was actually Martha Kent, and was forced to hand over the disc to her. Martha warned Tess about the bad choices she has made in her life and threatened her to stay away from Clark.
When Zod declared the war to the earth, Tess revealed she used a tracking device on John Corben to find the Fortress of Solitude and visited it, where Zod was trying to mess with the console. Looking for redemption for everything she had done, she attacked Zod with kryptonite brass knuckles and damaged his face. However, Zod managed to heal and attacked her with heat vision, burning half of her body.
Clark found her and rushed her to the hospital, where she told Clark about the crystal console the Kandorians built and how he could use it to activate the Book of Rao as her last chance for redemption. Later, she had a violent attack from her injuries, resulting in her death.
Once the doctors pronounced it and left the room, a mysterious old woman entered the room, shutting the door behind her.