Ep 21. Injustice

In the Smallville season 8 episode 21 titled "Injustice," a new superhero called The Black Canary arrives in town, causing Clark Kent to question her motives and identity. Meanwhile, Oliver Queen, also known as Green Arrow, leads a team of vigilante superheroes called the Justice League to take down a new supervillain named Tess Mercer who has taken control of LuthorCorp and is using it for evil purposes.

As Clark investigates The Black Canary, he learns that she is actually a woman named Dinah Lance who has come to Smallville to seek revenge on a man named Lionel Luthor, who she believes is responsible for her mother's death. Clark tries to convince Dinah to let go of her vengeance and use her powers for good, but she refuses to listen and continues on her path of revenge.

At the same time, Oliver leads the Justice League in an attack on LuthorCorp, but they are met with resistance from Tess Mercer and her team of superpowered soldiers. The battle escalates, and Clark is forced to step in and use his powers to protect innocent bystanders. However, he soon realizes that the Justice League's methods of vigilante justice are not in line with his own beliefs and decides to take a stand against them.

As the battle between the Justice League and LuthorCorp reaches its climax, Clark must make a difficult choice between saving innocent lives and stopping the Justice League from causing more harm. In the end, he is able to convince Dinah Lance to let go of her vengeance and join him in his mission to protect the world from evil. The episode ends with Clark and Dinah working together to bring down LuthorCorp and restore justice to Smallville.