On tonight's episode of "Arrow," a lot of familiar faces returned. While Roy Harper and Nyssa al Ghul's return were teased in the midseason trailer, two other characters came back to "Arrow" tonight, one of whom was previously thought to be dead. A key member of Team Arrow was also given a death sentence, a new villain was introduced, and a new running mate challenged Oliver's campaign for mayor. This episode was not only one of the most surprising episodes of the season, it was one of the strongest, and set up some major plot lines that may take the rest of the season to resolve.

The episode opened with an explosion of action as Team Arrow tracked down a thief with mad parkour skills. Oliver (Stephen Amell) cornered the theif, and was this close to catching him, until Thea (Willa Holland) blacked out and almost fell off the edge of the building. The thief got away, and Thea was taken home. Thea's blackout was caused by her unquenched blood lust, which started to take its toll once Damien Darhk's (Neal McDonough) powers wore off. Marlcom (John Barrowman) explained that if the Lazarus Pit can't take a life for a life through the blood lust, it will take Thea's life. Does that mean Thea will end up in the unmarked grave in a few months?

The thief was finally apprehended by Oliver, who tracked him down with Laurel (Katie Cassidy) and Diggle (David Ramsey.) The mystery tech stealer turned out to be Roy Harper (Colton Haynes.) After Oliver incapacitated Roy with an arrow and brought him back to the cave, Felicity (Emily Bett Rickards) noticed an optical lens in his eye that was transmitting a signal. When Roy woke up, he confessed to the team that he was being blackmailed by a criminal super hacker by the name of The Calculator ("Everwood's" Tom Amandes). The Calculator has a long history in DC Comics, most notably as an adversary for Oracle from the "Birds of Prey" comic. Felicity's code name may not be Oracle -- it's Overwatch -- but she is getting a lot of similar story lines to Barbara Gordon a.k.a. Oracle. The Calculator turned out to be a powerful villain tonight as he enacted a plan to wipe out all of Star City. Thankfully Felicity's hacking skills were able to beat him in time.

Another character who made a return appearance on tonight's episode was Nyssa al Ghul (Katrina Law.) Nyssa had been locked up under the orders of Malcom for destroying the Lazarus Pit earlier this season. Through the help of her father's loyal assassins, Nyssa was set free. She used her freedom to go to Japan in the hopes of finding a rare lotus flower. She found the flower, but it was being guarded by Tatsu Yamashiro, who was a part of the Crescent Order. In the comics, the "Order of the Crescent" was the highest honor that could be bestowed by the citizens of Kahndaq, the home of Black Adam. This might just be a coincidence, but more than likely it's a DC Comics Easter egg left by the "Arrow" writers. Tatsu (Rila Fukushima) was previously Maseo's (Karl Yune) wife and Oliver's good friend in season three, before Maseo was killed. Tatsu was carrying her sword and wearing her Katana mask (which is nearly identical to the "Suicide Squad" movie's version of Katana, who is being played by Karen Fukuhara.) Nyssa and Tatsu fought each other, until Nyssa made Tatsu an offer she could not refuse. Nyssa was not seen again until the end of this episode, but her return to "Arrow" could be a game changer, especially considering the trailer for next week's episode, "Sins of the Father."

With Thea incapacitated, Oliver had to decide if he was going to let her die, or if he was going to make a deal with Damien Darhk. When Oliver found out that Darhk's wife, Ruve Adams (Janet Kidder) was running against him, he contemplated trading in his mayorship for Darhk's healing powers. Malcolm Merlyn talked Oliver out of making that mistake, when he said that "showing love sometimes means doing nothing." That was a powerful statement considering Thea is Malcolm's daughter.

In the island flashbacks, Oliver was captured and tortured by Baron Reiter (Jimmy Akingbola. ) While he was being tortured, he passed out from the pain and started hallucinating. His hallucinations manifested Shado, returning actress Celina Jade, his former love and trainer on the island from season one and season two. Shado died on the island and clearly wasn't real, but she told Oliver she was there to help him let go of guilt. In terms of the season-long theme of "Arrow" this year, everything is about Oliver moving past his guilt, and letting go of the weight of world on his shoulders. Oliver has done a lot of wrong in his life, especially when he was a hired gun for A.R.G.US. in Hong Kong, but that's not who he is anymore. He actually has a chance to completely change and become a true hero this season, if he can forgive himself for his past. The writer's made a powerful choice in using Shado as Oliver's guide to healing, considering the biggest guilt he carries is choosing Sara's life over hers.

Felicity had her own demons the face this week, as her company, Palmer Tech, wanted to launch their newest super battery. Felicity's tech genius co-worker Curtis Holt (Echo Kellum) was back this week, and his presence and encouragement was needed when Felicity thought about backing out of the presentation. With Felicity in a wheelchair, and with her focus on Team Arrow, she wasn't at the top of her game emotionally. Her board tried to get her to back down, but Curtis reminded her that she was strong and fierce and should do the presentation anyway. So she did, and she was a hit. Felicity was met with a surprise though -- her father showed up after she finished her speech. She recognized him right away, but what she didn't know was that the super hacker she was fighting earlier -- The Calculator -- was the same man! Felicity's dad is a super villain! What's great about Tom Amandes playing her dad, is that he was Emily VanCamp and Chris Pratt's dad on "Arrow" EP Greg Berlanti's first CW show, "Everwood." Now all of Amandes' TV kids have joined the superhero universe, as Emily VanCamp is playing Sharon Carter/Agent 13 "Captain America: Civil War," and Chris Pratt is playing Peter Quill/Star-Lord in "Guardians of the Galaxy Vol. 2."

The episode concluded with Oliver turning down the meeting with Darhk, Roy spending time with Thea, and Nyssa showing up to offer to cure Thea if Oliver kills Malcolm Merlyn. Nyssa's plan is bold and dangerous. Thea's life hangs in the balance, and all Nyssa wants is her revenge. It was great to see everyone back on the show this week -- Katrina Law, Celina Jade, Rila Fukushima, Colton Haynes -- it was especially nice to see Colton Haynes. His scenes with Thea were so touching and emotional in this episode. You can tell they really love each other and it would be heartbreaking to watch either of them die this season. Hopefully Thea's Lazarus sickness can be cured and they can be together as Speedy and Arsenal.

Next week, Nyssa's plan to save Thea and reclaim her place as Heir to the Demon kicks into gear. Oliver must confront Malcolm/Ra's al Ghul again, in order to save Thea's life. Will Malcolm forfeit the ring of Ra's to save her? Find out next week on The CW, Wednesday at 8/7C.