So what does that key unlock? You'll find out in Episode 8. "The key is very significant. It definitely has something to do with where this boy has been," creator and executive producer Jenna Bans tells TVGuide.com. "We will definitively find out not only what that place is, but it will sort of fill in a big piece in terms of Adam's identity. It's definitely something that's very important to him and it's very important to his story as a character too."
A couple things you can rule out: The key does not open anything we've already seen, nor does it have anything to do with the DNA testing lab that confirmed his identity and is working on the sex offender tracking chip Claire (Joan Allen) has proposed. "It's a key to a place that basically makes a lot of things clear about the question we're asking: Is it or isn't it him?" Bans says. "By the time we see what that place is, we've already answered that question, so it's sort of a nice next piece of the mystery filled in. ... It's not like the key opens a hatch that leads to 10 more mysterious things. The key opens basically an answer."
The second piece of the puzzle will come next week's episode, when Adam learns another piece of intel about himself. "[That new information] has to do with when he finally uses the key in Episode 8 to get to where he needs to get to," Bans teases.