Concern for the crocodiles' welfare prompted an investigation by the Royal Society for the Prevention of Cruelty to Animals (RSPCA), which ultimately took no action against APF.

[3][4] Along with "The Cham-Cham", the next episode to enter production, it went over-budget, causing the final instalment of Thunderbirds Series One ("Security Hazard") to be re-written as a clip show to lower costs.

A businessman, Blackmer, visits the reclusive Dr Orchard, a scientist who lives in a dilapidated house on the Ambro River.

From the local plant Sidonicus americanus, Orchard has developed a food additive called "theramine" that increases the size of animals.

When a storm forces Blackmer to stay at the house overnight, Culp decides to steal the theramine and sell it to the highest bidder.

The house is quickly surrounded by three giant alligators that repeatedly hurl themselves at the building with Orchard, Blackmer, McGill and the housekeeper, Mrs Files, trapped inside.

Threatening to empty the entire theramine vial into the Ambro unless he is given safe passage upriver, Culp sets off in Blackmer's boat.

[9] Writer Alan Pattillo, who according to special effects supervisor Derek Meddings "had tried to come up with the most nightmarish rescue situation he could",[10] had wanted to direct the episode as well.

[12] The production overran its one-month schedule, forcing the crew to work extra hours, and sometimes long into the night, to finish the filming.

[11] Special effects assistant Ian Wingrove remembered that the episode's complex technical aspects had the crew "[working] day and night ... through a weekend".

These were acquired from a private zoo in the north of England to double as the enlarged alligators on the episode's scale model sets and water tanks.

[6] The animals were unpredictable and difficult to control, either basking in the heat of the studio lights or disappearing into the tanks for hours at a time.

He ended up taking his annual leave and coming to the studio to work for us, and he was personally giving the crocodiles electric shocks.

Effects director Brian Johnson and several other crew members refused to take part on animal welfare grounds.

[6] Camera operator Alan Perry did not remember any of the crocodiles being mistreated; series supervising director Desmond Saunders, however, claimed that more than one specimen died of pneumonia after being left in an unheated tank overnight.

During a promotional photoshoot featuring Lady Penelope (who does not appear in the episode), one of the animals attacked the puppet and destroyed one of its legs.

[8][16][18][20] Meddings wrote of the incident: "My crew never saw me move as fast as I did to get out of the tank when I pulled the rope and realised the creature was free.

This led the writers to re-work the final episode of Thunderbirds Series One ("Security Hazard") as a clip show to reduce costs.

[2] During that channel's 2000-2001 Thunderbirds re-run, the episode became the eleventh to be repeated when it replaced "Brink of Disaster", which along with "The Perils of Penelope" had been postponed until the end of the run due to similarities between the story and real-world events (both episodes feature dangerous situations involving trains and 2000 had seen several major railway accidents, most notably the Hatfield rail crash).

[27] Lew Grade, head of distributor ITC, expressed great satisfaction with the filming during a visit to APF Studios in 1965.

[16] Stephen La Rivière considers the story one of the most unusual of the series,[5] while Peter Webber of DVD Monthly magazine calls the episode "just insane".

[4] Mark Pickavance of the website Den of Geek criticises the footage from a visual standpoint, arguing that the use of scale sets with young crocodiles, "shot in super close-up to make them seem huge", does not produce a convincing illusion of giant alligators.

[1] Author Dave Thompson compares the giant reptiles to Swamp Thing, a superorganism featured in the DC Comics Universe.