Professor Rivers of the Pharos Institute invites Sarah Jane to participate in her investigation of Ashen Hill Manor. The estate has been plagued by rumours of hauntings and mysterious disappearances for centuries, dating all the way back to 1665, when it was home to the purported alchemist, Erasmus Darkening. Sarah Jane is skeptical at first... until Professor Rivers vanishes as well. With spirits roaming the secret passages that riddle the house, and something malevolent stalking the grounds after dark, Sarah Jane and her friends must find out what really happened in Ashen Hill Manor three hundred and fifty years ago.

Ever since he was invited to contribute to the first season of The Sarah Jane Adventures, lead writer Phil Ford had been keen to tackle a haunted house story. In addition to taking the show in an interesting new direction, he also felt that it could be a budget-conscious exercise, since it could predominantly make use of a single setting with few computer-generated effects. It was finally decided that such a narrative should be developed as Ford's second script for the programme's third season, following the premiere serial Prisoner Of The Judoon. Originally, the spooky story was pegged to be the year's second adventure; however, when the serial originally intended to be the finale -- The Wedding Of Sarah Jane Smith -- had to be shifted to an earlier position due to the availability of Tenth Doctor actor David Tennant, Ford's script was bumped to the fourth spot.

Several regular characters would be missing from the story, which eventually gained the title The Eternity Trap. Originally, Ford had been asked to limit the involvement of Luke Smith in the action, to facilitate Tommy Knight's GCSE exam preparation. With the delay in its production, however, The Eternity Trap would now be filmed during the exams themselves; as such, Luke had to be completely removed from the story. Also intentionally omitted was Mr Smith, given that no filming was planned for Upper Boat Studios, and no material could be set on Bannerman Road due to the unavailability of the usual Clinton Road location in Penarth during the corresponding filming dates. Meanwhile, K·9 was absent because his return in the season's original narrative sequence had taken place after the events of The Eternity Trap.

With so many characters unavailable, Phil Ford elected to bring back Professor Rivers, played by Floella Benjamin

For a time, it was even thought that Ford's serial might have to make only sparse use of Sarah Jane in order to provide Elisabeth Sladen with a mid-season break. However, this requirement was subsequently shifted to Mona Lisa's Revenge, the next story to go before the cameras. With so many characters unavailable to him, Ford elected to bring back Professor Rivers, played by Floella Benjamin, the Pharos Institute scientist whom he had created for 2007's The Lost Boy and used again in the following year's The Day Of The Clown. Forgetting that the script for The Lost Boy had given the character the first name Jane -- a fact not mentioned on screen --- Ford now rechristened her Celeste.

Ford had hoped to be able to write his script for a specific filming location, but this did not prove possible. Consequently, changes had to be made once the main venue was finally identified as Dyffryn Gardens. Located in St Nicholas, the estate was much newer than the Civil War-era Ashen Hill Manor of the script. In fact, it had been constructed in the late nineteenth century by shipping and coal magnate John Cory, and was now operated by the National Trust and chiefly known for its botanical gardens. Because the property was landlocked, Ford relocated action originally set in a boathouse to a pavilion. He also dropped the notion of a gate-like structure at the foot of the main staircase which would have played a role in the defeat of Erasmus Darkening, and instead took advantage of the presence of the large, ornate crest of the Cory family.

The Eternity Trap was directed by Alice Troughton, who had just finished work on The Mad Woman In The Attic. Production in June 2009 began with five days at Dyffryn Gardens, on the 3rd, 4th, 8th, 9th and 10th. Anjli Mohindra found herself appropriately unsettled by the story's spooky atmosphere -- especially since the grounds of Dyffryn Gardens were allegedly haunted by seventeenth-century explorer Sir Thomas Button, who had lived in an earlier manor house on the same site. The only other venue used by Troughton's team was Caerphilly Castle in Caerphilly, for material in Darkening's secret laboratory and the passages which led to it. Recording there took place on June 11th and 12th. Cast and crew then returned to Dyffryn Gardens from the 15th to the 18th to complete The Eternity Trap.

• Doctor Who Magazine Special Edition #28, 13th July 2011, “Episodes 3.7/3.8: The Eternity Trap” by Andrew Pixley, Panini Publishing Ltd.