At the start of The Defenders (spoilers for Episode 1 ahead), Luke Cage is on his way out of prison. He's barely had time to, uh, christen his relationship with Claire Temple, when Detective Misty Knight walks in with a new problem about a young neighborhood man who was mysteriously killed. And she's worried that the rest of Harlem's youth might be in danger too. Who killed Sean on The Defenders? The answer may lead to a city-wide threat.
It turns out that Sean is the sister of someone Luke's crossed paths with before: Candace, the young woman who almost testified against the villainous Mariah in Luke Cage. "I think someone's been using Harlem's youth for what can only be described as 'late night duty,'" explains Misty. "Seven in the last few months," she later elaborates. "Always the same story. Harlem 20-something moves his mom out of the projects, and into a nice home in New Rochelle. Down payment in cash ... No guns, no drugs, just a new job off the books. Doing what, nobody knows." Misty urges Luke to connect with Cole, Sean's brother — to try and save him from the same fate.
Despite Claire's plea that he take some time to take care of himself, Luke quickly catches up with Cole, who won't say much about Sean's new job, which might be his new job too, for that matter. Luke is quick to notice details like the expensive new shoes lined up on Cole's shelf, but Cole won't tell Luke anything about where he's getting his money, even after Luke offers to help keep him safe. Cole only tells him it's too late, and asks him to leave. He says, hauntingly, "The only way to help me is to make me bulletproof too."
Spoilers follow for episode 2 of The Defenders. Of course, Luke's not going to let it go — so he continues to investigate, heading over to Trouble and a Pair of Dice, the new criminal hotspot in Harlem, as per Claire. He wastes no time shaking down a local grifter, who tells him that a new guy in town that they're all calling White Hat is the man he's looking for. He's a "real deal African brother" with a "white suit, Panama hat, alligator shoes" — and, perhaps most importantly, the man tells Luke where White Hat can be found. Luke heads to the address and finds White Hat ushering some kids into a van — including Cole.
Yep, it turns out that White Hat is with the Hand — and Cole's been working night clean-up for them. And knowing the kind of situations that the Hand usually find themselves in, this clean-up work is probably extensive, bloody, and the kind of thing you keep your mouth shut about. And you know who else is actively seeking out information on the Hand? That's right, Danny Rand.
Coincidentally, Danny's search for the Hand has led him to the very same people Luke is trying to protect. Luke walks in on Danny beating up Cole for information at the end of episode 2, and, suffice it to say, it's not the best way for the two heroes (who have a long history as friends in the comics) to meet. Luke points out to Danny that kids like Cole are the victims in this situation. They're being taken advantage of by an organization that knows they are desperate for resources and to provide good lives for their families.
Perhaps Sean realized that the work that he was being asked to do was not worth the New Rochelle house and the nice shoes and the Hand took him out before he could talk. Regardless of who actually cut his life short, Sean's death is very important to the fabric of the show: he helps bring together Luke and Danny together; and he shows how the Hand is infiltrating New York City via its most vulnerable citizens.