Thunderbirds Are Go is a 1966 British science-fiction puppet film based on Thunderbirds, a Supermarionation television series created by Gerry and Sylvia Anderson and produced by their company Century 21 Productions. Written by the Andersons and directed by David Lane, Thunderbirds Are Go concerns spacecraft Zero-X and its human mission to Mars. When Zero-X suffers a malfunction during re-entry, it is up to life-saving organisation International Rescue, supported by its technologically-advanced Thunderbird machines, to activate the trapped crew's escape pod before the spacecraft hits the ground. Filmed between March and June 1966 at Century 21's studios on the Slough Trading Estate and on location in Portugal, Thunderbirds Are Go features guest appearances by puppet versions of Cliff Richard and The Shadows, who also contributed to the film's score. It was the first film to be shot using an early form of video assist called "Add-a-Vision". The film's special effects sequences, directed by Derek Meddings, took six months to complete. Although early reviews praised the film as a successful cinematic transfer of the TV series, Thunderbirds Are Go drew a lukewarm public response and proved to be a box office failure. Later reviews would criticise the film for its minimal characterisation, lengthy effects shots, and inclusion of a fantasy dream sequence centring on Richard and The Shadows. Surprised by the film's underperformance, and confident that Thunderbirds still had cinematic potential, distributors United Artists ordered a sequel, Thunderbird 6. However, this too received a mediocre critical and commercial response and caused the franchise to be abandoned until the early 2000s. Zero-X later appeared in the first episode of Captain Scarlet and the Mysterons, the Andersons' follow-up to Thunderbirds, while tie-in publication TV Century 21 ran a Zero-X comic strip until 1969.