"Treble Cross" is the twenty-second instalment of Captain Scarlet and the Mysterons, Written by Tony Barwick and directed by Alan Perry, it was first broadcast on 23 February 1968 on ATV Midlands.
In this instalment, Captain Black engineers the death and reconstruction of a test pilot as part of a plot to destroy the world capital, Futura City. However, his victim is revived in a hospital and under Spectrum's supervision impersonates his Mysteron replacement in a bid to trap Black.
World Air Force test pilot Major Gravener is being driven to Slaton Airbase when Captain Black, approaching from the opposite direction in a truck, extends a beam with false headlamps into the road to dazzle Gravener's chauffeur, Harris. Fearing a collision, Harris plunges his car into a lake and both he and Gravener drown. The clinically-dead Gravener is discovered by two passing doctors and taken to Slaton Hospital, where he is revived with the aid of a stasis and recovery unit.
The next day, a Mysteron reconstruction of Gravener travels to Slaton Airbase and commandeers an XK-107 bomber armed with a nuclear warhead. When word reaches the base of Gravener's accident, the command staff realise that the pilot is an impostor and block the runway just as he is taking off. The XK-107 crashes and explodes, killing the reconstructed Gravener.
Spectrum makes a connection between the attempted theft of the XK-107 and the latest Mysteron threat to destroy the world capital, Futura City. Briefed by Captains Scarlet and Blue, the original Gravener leaves in a second XK-107 in an attempt to impersonate his Mysteron reconstruction. As hoped, Captain Black telepathically contacts the aircraft: he instructs Gravener to land 30 miles outside Futura City at the disused Weston Airstrip, where his truck is waiting. Scarlet speculates that the purpose of the rendezvous was to transfer the warhead to a road vehicle, after which Gravener's reconstruction was to have continued flying the XK-107 as a decoy while the warhead was driven into Futura City and detonated.
Spectrum forces, including Scarlet and Blue in a Spectrum Patrol Car, converge on the airstrip to apprehend Black. The truck, presumed to be driven by Black, attempts to escape but crashes into a bunker. However, Scarlet, Blue and the newly-arrived Gravener are shocked to find that the deceased driver is not Black, but a Mysteron reconstruction of Harris. Although the Mysterons were unable to distinguish between the two Graveners, Black's "sixth sense" has enabled him to evade Spectrum once again.
"Treble Cross" was filmed over two weeks in September 1967, during which time Century 21's next Supermarionation series, Joe 90, entered pre-production.
The scale model representing Slaton Hospital was previously used as the microbiological laboratory in "Place of Angels". For realism, during the filming of the revival scenes set inside the hospital the front of the Major Gravener puppet was fitted with an airbag to make it appear that the character's chest is rising and falling.
A 40-second montage of clips showing Spectrum ground forces converging on Weston Airstrip is made up of footage duplicated from "Manhunt", another episode in which Spectrum attempts to apprehend Captain Black.
"Treble Cross" is one of several Captain Scarlet episodes written by Tony Barwick that include a mention of 10 July. Barwick liked to insert references to this date as it was his birthday.
Anthony Clark of sci-fi-online.com considers this episode's premise "more interesting" than that of "Fire at Rig 15" but describes "Treble Cross" overall as "a little slow".
Media historian Nicholas J. Cull argues that "Treble Cross" is one of a number of Captain Scarlet episodes that demonstrate writer Tony Barwick's partiality to stories highlighting the dangers of nuclear technology. He views the plot involving the attempted theft of a military aircraft carrying a nuclear warhead as an example of Barwick employing his "favourite device".