WARNING: The following article contains spoilers for the reboot of Lost in Space, steaming now on Netflix.

A reimagining of the 1960s sci-fi adventure of the same name, Netflix's Lost in Space borrows the premise and characters of the Irwin Allen classic, but it also makes use of one of the original stars in a clever way. A child actor whose resume already included The Twilight Zone, The Adventure of Ozzie and Harriet, and Dear Brigitte, Bill Mumy was propelled to stardom with his role as Will Robinson, the 9-year-old prodigy so frequently warned of danger by the protective Robot. Now 64, Mumy returns to the property for a cameo, not as Will Robinson, of course, but in another, recognizable role.

It's well-publicized that Parker Posey was cast in the reboot as a gender-swapped Dr. Smith, who, as played by Jonathan Harris in the '60s series, wavered between villainous saboteur and campy comedy relief. However, details of her role have remained secret, with reviews making only vague reference to her character's shady past and nefarious motives. That's because, in the closing moments of the premiere episode, "Impact," we learn she's not really Dr. Smith, but rather an opportunistic impostor.

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Lost in Space is set 30 years in the future, where Earth is on the verge of becoming uninhabitable, and the best and bright families are selected for an interstellar voyage aboard The Resolute to colonize a new world in the Alpha Centauri system. The previous missions, we assume, were completed with little to no problems, but on the 24th, The Resolute experiences a breach, and the families are ordered to abandon the mother ship aboard their smaller Jupiter vessels, until the damage can be repaired. But this isn't a mere accident, like a collision with space debris, but rather an attack that leaves dead and wounded in its wake.

In the mayhem, Posey's character runs into a wounded colonist, and offers to help (well, make that "help"). There's a lot going on in this scene, so you can be forgiven if you don't immediately recognize the bleeding man as Mumy. But director Neil Marshall (Game of Thrones, Hellboy) makes sure you recognize his name:

So, yes, Bill Mumy, who rose to fame as Will Robinson on the original Lost in Space, portrays Dr. Zachary Smith on the 2018 reboot. Or at least we assume the "Z" stands for Zachary.

Pretending to help Dr. Smith, Posey's character instead helps herself to his jacket, whose I.D. tag includes a security chip that gains her access to his family's ship, Jupiter 18, and therefore escape from the danger aboard The Resolute. Unable to pilot the vessel herself, she assumes the air of authority that comes with her new identity, and offers passage to the far more capable mechanic Don West (Ignacio Serricchio) and his colleague Tam (AnnaMaria Demaria).

Although Mumy's Dr. Smith is left behind on The Resolute, that's not exactly the last we see of him. Like the Robinsons' Jupiter 2, the Jupiter 18 crashes on a dangerous planet, where Posey's character continues the charade. Discovering the Smith family photo album among the ship's debris, she quickly buries it, but not before giving us a good look at Mumy.

Unfortunately for "Dr. Smith," she's unable to dispose of all of the evidence, and when Don and a group of other colonists return to Jupiter 18 in Episode 6 to siphon its fuel, he discovers a hospital I.D. badge depicting the real Dr. Smith -- that is, Mumy's character.

While we've come to expect reboots to feature original cast members, those cameos can often be jarring and gimmicky, with the story pausing as if to wink, and give the audience sufficient time to recognize the actor. On Lost in Space, however, Mumy's cameo is seamless, and integral to the story, even if the viewer doesn't realize who's playing the real Dr. Z Smith.

Mumy's career continued beyond the cancellation of the original Lost in Space in 1968, as an actor and voice actor (Babylon 5, Biography), one-half of the comedy rock duo Barnes & Barnes (best known for the song "Fish Heads"), and as a fixture of the sci-fi community. He and former co-star Jonathan Harris turned down cameos in the poorly received 1998 film adaptation of Lost in Space, which featured appearances by original cast members June Lockhart, Mark Goddard, Angela Cartwright and Marta Kristen, with Dick Tufeld reprising his voice role as Robot.

Steaming now on Netflix, the 10-episode first season of Lost in Space stars Toby Stephens, Molly Parker, Maxwell Jenkins, Taylor Russell, Mina Sundwall, Ignacio Serricchio and Parker Posey.