'Supergirl': Why Lex Luthor Returns in "O Brother, Where Art Thou?"

It's no secret on Supergirl that Lex Luthor is bad news. His sister, Lena, has spent a lot of time and effort trying to divorce herself form her big brother's criminal shadow on The CW show. Yet, tonight's episode sees Lena welcoming Lex into her home when he becomes too frail to remain behind bars. But how, exactly, did the formidable Lex Luthor end up in this predicament?

Spoilers for tonight's episode of Supergirl, "O Brother, Where Art Thou," below.

When Lex is brought to Lena in tonight's episode, it's because the is dying of cancer. Lex being taken from prison is something of a variation of compassionate release -- he's not exactly done with prison, as he's monitored by guards and is handcuffed to his wheelchair -- in which he's come to Lena in an attempt to find a cure to save his life.

You see, Lex isn't suffering from an ordinary type of cancer. Lex's life-threatening illness is caused by kryptonite poisoning, something that makes sense as he has had extreme exposure to it due to his obsession with Superman. As one might guess, kryptonite-related cancer isn't something that is well-treated by conventional medicine thus making Lena his only hope.

The idea that Lex has this unique form of cancer is one that fan will recognize as being inspired by comics. In Action Comics #600, Lex is shown to be suffering from severe cancer as a result of the long-term kryptonite radiation exposure prompted by his wearing of a kryptonite ring -- an item he wore to show that even Superman couldn't touch him. While it was first assumed that human beings weren't impacted by kryptonite radiation, that turned out to be wrong. Lex's hand had to be amputated due to the cancer, but it was too late. The disease had already spread to terminal levels.

Of course, in comics, Lex doesn't seek out a miracle serum built from another Kryptonian rock. Instead, he fakes his own death in order to create a cover for having his brain removed from his dying body so that a clone body could grow around it. Lex returned, in his shiny new body, pretending to his previously-unknown illegitimate son, Lex Luthor II. It's a cunning plan that doesn't quite go the way Lex expects it to -- the clone body eventually begins to deteriorate and age rapidly before he ends up literally selling his soul to the demon Neron for health.

While it's not likely that Supergirl will see Lex cloning himself and selling his soul to a devil, his cancer diagnosis certainly will give Lena a hard push towards finding a way to make her super serum -- as derived from the Harun-El she's been working with -- a functioning cure for disease. Not that Lena will need much of one: at the end of last week's episode, James was shot and left for dead in his office. Lena's ex might need the healing properties of her experiments, too.

Supergirl airs Sundays at 8 p.m. ET on The CW.