The CW's "Arrow," Season 4, episode 5 features the highly anticipated crossover between the Green Arrow (Stephen Amell, right) and John Constantine (Matt Ryan, left) from NBC's short-lived series "Constantine." CW

Season 4 of the CW’s “Arrow” has been slowly introducing the higher-level concepts of mysticism that runs through the DC Universe. In episode 5, a new friend came to Star City to help Team Arrow solve its strangest problem yet, thanks to a special crossover event with NBC’s “Constantine.”

The episode opens with Sara Lance (Caity Lotz) interrupting a pair of very bad men doing very bad things to a woman in an alleyway by brutally killing them. It would be a heroic moment, if she hadn’t also killed the woman. Since she escaped from her sister’s not-so-smart holding cell, the soulless creature has been on a bit of a murder spree. Now, the news of her resurrection can’t be kept a secret for much longer. However, Oliver Queen (Stephen Amell) takes some time to figure it out given that he’s distracted by a mayoral campaign. His sister, Thea Queen (Willa Holland), brings in a political strategist who bluntly tells Oliver that, to win the hearts of the people of Star City, he needs to distance himself from his old playboy lifestyle. The biggest manifestation of that is his relationship with Laurel Lance (Katie Cassidy) and the scandal surrounding the sinking of the Queen’s Gambit.

Before Oliver’s decision about distancing himself is made clear, the episode cuts to this season’s flashback storyline, which picks up immediately after the cliffhanger last week. In it, one of the soldiers for whom Oliver had been working discovers his ARGUS-issued communications equipment. The soldier takes him at gunpoint to the boss, Baron Reiter (Jimmy Akingbola), only to find his chief dealing with a much bigger problem than that associated with some perfectly explainable communications equipment: He’s found a spy searching their camp, a well-dressed man named John Constantine (Matt Ryan). The title character in the NBC series crossed over to the CW show to finally show Oliver the world of magic.

Constantine gets away from his captors and forces Oliver to take him to a special spot on the island of Lian Yu. The two form your typical good-guy bond along the way. Once they arrive, Constantine reveals the world of magic to Oliver for the very first time. To the hero’s credit, he takes it very well and follows Constantine into a secret underground lair that contains a scepter with a spell on it. It’s booby-trapped, of course, and Oliver has to rescue his new friend just in time, thus proving he’s not like the men he appears to be hanging with. Constantine thanks him and tells him he owes him a favor.

Back in the present day, Oliver finally discovers Laurel’s secret when Team Arrow goes looking for the person responsible for the recent string of murders in town. He sees Sara for the first time and puts everything together about her resurrection with the Lazarus Pit. He makes the usual rounds of Laurel, Thea and Felicity Smoak (Emily Bett Rickards), with each giving him advice and answers about what happened.

However, it isn’t until he goes to Capt. Quentin Lance (Paul Blackthorne) that things begin coming together. Lance tells him that the city’s resident crime boss, Damien Darhk (Neal McDonough), has enlisted his help to infect a federal data server with a virus. Oliver admits he can’t help but that his partner, John Diggle (David Ramsey), can. Together, they break into the server room to discover that the virus was meant to delete a mess of files that may involve Darhk’s organization, HIVE. In carrying out the work, Diggle notices that his murdered brother has a file among those deleted.

Meanwhile, Oliver and Felicity figure out a pattern to Sara’s apparently random attacks. Each of her victims looks an awful lot like Thea, the girl who killed Sara in the first place. Sure enough, the episode cuts to the demented Sara attacking Oliver’s sister in her apartment. Thea gets away, but has to be hospitalized because of her injuries. While there, she confesses that the only reason her bloodlust side effects from the Lazarus Pit have seemed so calm lately is due to the fact she satiated it by killing two men a few weeks ago. She knows that all Sara needs to do is kill her and she’ll feel better, but Oliver has a better idea. He calls on his old friend, Constantine, who makes the trip to Star City to repay Oliver for saving his life back on Lian Yu. Constantine shows up just after Team Arrow manages to capture Sara by using her bloodlust for Thea as bait.

Together, they perform a ritual that sends Oliver, Laurel and Constantine to the realm where Sara’s soul is being held captive. After a pretty epic sword fight between Constantine and a demon, the rescue of Sara’s soul is complete, and she returns to Star City completely restored. Exception for the few lives that she ended along the way, things wound up all right for the gang just this once.

To alleviate the risk of letting the episode conclude on too much of a high note, Lance goes to Diggle’s house to reveal some bad news. He asked Darhk about his brother and learned that HIVE killed him in an effort to wipe out competing war profiteers in Afghanistan. That’s right, the brother who Diggle has spent the whole series trying to avenge was nothing more than a vicious criminal.

The episode ends on a small Ray of hope (pun intended) as Felicity and her new Terrific best friend (pun intended again), Curtis Holt (Echo Kellum), discover that there’s something buried in the last audio recording Ray Palmer (Brandon Routh) made before he allegedly died in a fiery explosion. Maybe the former superhero isn’t so dead after all.