World Air Force test pilot Major Gravener is being driven to Slaton Airbase by car 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 swerves the 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 personnel 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 connects the attempted theft of the XK-107 to a 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 shown to be waiting. Scarlet deduces 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 ground forces, led by Scarlet and Blue in a Spectrum Patrol Car, converge on the airstrip to capture Black. The truck attempts to escape but crashes into a bunker, killing its driver. However, Scarlet, Blue and the newly arrived Gravener are shocked to find that this man 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.
Production
"Treble Cross" was filmed over two weeksin September 1967, during which time Century 21's next Supermarionation series, Joe 90, entered pre-production. The scale model representing Slaton Hospital previously appeared as the microbiological laboratory in "Place of Angels". For realism, during the filming of the revival scenes the front of the Major Gravener puppet was fitted with an air bag to make it appear that the character's chest is rising and falling. Footage showing the Spectrum forces converging on Weston Airstrip is 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 to include a mention of 10 July. Barwick liked to insert references to this date as it was his birthday.
Reception
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". Shane M. Dallmann of Video Watchdog magazine calls it "less amusing but far more intelligent" than "Flight to Atlantica". Media historian Nicholas J. Cull believes "Treble Cross" to be one of several Captain Scarlet episodes that demonstrate writer Tony Barwick's interest in the dangers of nuclear technology. He regards the plot involving the attempted theft of a nuclear warhead as an example of Barwick re-using his "favourite device".
