Altered Carbon has never been a show to shy away from initialisms, jargon, or confusing concepts. Even a succinct recap of the show is a dense knot of story threads, politics, and technological innovation. And that’s just what the writers make explicit.
There are other things the show implies, brushes past, or that the characters mention only in passing that are even less clear. Let’s talk about something that comes up in both seasons: double sleeving.
DHF, stacks, sleeves, needlecasting, and backups
The basics of Altered Carbon go like this: humans have discovered a way to store their consciousnesses as digital human freight (DHF) on stacks (those silver discs in their necks) that are implanted into sleeves (bodies), making them basically immortal.
Having a person’s entire personhood in a digital format (as DHF) is only step one in Altered Carbon’s process. A DHF is just raw data — a file. When it’s stored on a physical disk, a stack, it’s removed from wherever it was stored, and exists only on the stack.
DHF is transferred between stacks and sleeves via needlecasting. Think of this as a faster-than-light wifi. Needlecasting allows for interstellar travel (assuming there’s a transmitter and receiver on either end). A person’s DHF is cast from one stack into another, and that stack is implanted into a sleeve of some sort — an organically grown clone sleeve or a synthetic one. We meet the bounty hunter Trell in a synthetic (synth) sleeve in episode 1 of season 2, for example. Synths are the cheaper option, while the wealthy (meths) tend to grow a new organic sleeve for their travel.
Backups are slightly different. A backup is a stored copy of a DHF — a snapshot of that person’s consciousness. In season 1, Laurens Bancroft created backups every 48 hours. With his death, his most recent backup was used to restore him to life, but he didn’t remember anything from the previous 48 hours — the time between his last backup and when he reawakened. In season 2, the process seems a lot more efficient, with backups and transfers happening in nearly real time.
The important thing is that there’s only ever one copy of a DHF in one sleeve at any given time. In fact, it’s a federal crime to have a DHF in more than one sleeve at the same time — which is referred to as double sleeving — and is punishable by real death (destroying a person’s DHF permanently).
Illegal isn’t the same as impossible, though. In fact, double sleeving comes up several times across both seasons of the show. In episode 1 of season 1, we meet Dimi the Twin whose name is an acknowledgment of his crime. Double sleeving also plays a big part in the resolution to season 1.
Double sleeving in season 2 (SPOILERS)
Season 2 takes the concept of backups and double sleeving, and combines them in a new way.
We learn that the UN Protectorate (the government that encompases Earth where season 1 takes place, the season 2 setting of Harlan’s World, and the other settled worlds), and its military (the Colonial Tactical Assault Corps or CTAC) makes backups of most of its valuable soldiers.
In episode 4, we find out that former CTAC soldier Takeshi Kovacs is one of those valuable soldiers. The copy that the Protectorate has, though, is from over 250 years ago — before he found his sister and joined Quellcrist Falconer and her Envoys.
“Because of stack technology, because the mind or the body can exist separate from one another and be copied, a person can be copied, and that obviously opens up a whole bunch of story possibilities that don’t exist in every show that you write on,” showrunner Alison Schapker told Polygon about the big twist. “And so we love exploring that in Altered Carbon, and we loved in season two exploring that, for Kovacs, he would have to face a younger version of himself. And not just a younger version, but a version of himself before he met Quellcrist Falconer, a version that was loyal to the Protectorate, who was loyal to his commander, and had not yet experience the power of connection and love for this revolutionary leader and a woman who changed his life.
Colonel Carrera (formerly Jaeger), calls this version of Kovacs “Evergreen.” He’s his secret weapon. Carrera double sleeves this old version of Takeshi (Takeshi Prime) into a clone of Kovacs’ birth sleeve (played by Will Yun Lee), and sends him to kill the current version of Takeshi Kovacs (played by Anthony Mackie).
Even as part of a military operation, double sleeving is illegal and punishable by death for Carrera and both versions of Kovacs. At the end of the final episode of the season, Kovacs Prime argues that there’s only one DHF and one sleeve remaining, so it’s a moot point.
The big reveal during the show’s final scene, though, is something that will undoubtedly come up again in a season 3. But in season 2, showrunner Schapker says it was a just a key part of creating inner-conflict for Kovacs.
“I think for all of us, and we have been a watershed moments in our lives and we also have that desire to if I could talk to my younger self, or what I would teach, or what I wish my younger self knew, or if I only, you know, that idea of like integrating selves over time, like we’re all kind of walking around with versions of our younger selves. And so we really tried to give Kovacs an arc where he had to wrestle with a version of himself again and integrate a central conflict in his identity by coming to terms with who he was again. And also having guilt over what he had done in relation to his sister, which is part of what haunts him in season 2.”