The Doctor brings Martha home, on the day after she joined him in the TARDIS. Martha learns that her sister, Tish, has been working for the elderly Professor Lazarus, who has invented a machine which will change the world. Intrigued, the Doctor accompanies Martha to the unveiling of Lazarus' device, only to earn a frosty reception from her mother, Francine. Lazarus announces that he has found a way to reverse ageing and demonstrates it on himself, apparently restoring his youth. But the Doctor discovers that Lazarus' DNA is now unstable, and is trying to transform him into something monstrous.

When Doctor Who returned to television in 2005, one of the viewers it impressed was Stephen Greenhorn. An established scriptwriter, Greenhorn happened to be adapting the novel Wide Sargasso Sea for Julie Gardner, the BBC Wales Head of Drama who was also an executive producer of Doctor Who. In the spring of 2006, he met with Gardner and script editor Simon Winstone to express his admiration for the programme's revival, and his interest in working on it. This led to a discussion with showrunner Russell T Davies, who surprised Greenhorn with an offer to write for the programme's twenty-ninth season. In particular, he wanted an episode set on modern-day Earth involving a “mad scientist” character. A point of reference quickly became the scientific masterminds of Marvel Comics' superhero range, such as Spider-Man foes Doctor Octopus and the Green Goblin. Greenhorn also sought inspiration from tales of experiments gone horribly wrong featured in films such as The Fly (originally released in 1958 and remade in 1986), and classic stories like 1886's Strange Case Of Dr Jekyll And Mr Hyde by Robert Louis Stevenson.

Several of Greenhorn's ideas had to be discarded because they were too similar to concepts being developed for the first season of the spin-off series Torchwood. He also wanted to set his narrative around the Thames Flood Barrier, only to learn that it would already feature prominently in the 2006 Christmas special, The Runaway Bride. A further suggestion, concerning the development of an invulnerable synthetic skin, was dropped because Davies feared that it might coincide with the portrayal of the villainous Venom in Spider-Man 3. The highly-anticipated movie was due for release in May 2007, around the same time that Greenhorn's episode would likely air, although its portrayal of the character would ultimately hew closer to Venom's comic book roots as an alien parasite.

The aged scientist was first given the pulpish name Professor Anger

Finally, Greenhorn hit upon the notion of an aged scientist trying to make himself young again. This character was first given the pulpish name Professor Anger, and subsequently became Professor Lazarus, after the man raised from the dead by Jesus Christ in the Gospel According To John. Accordingly, the episode gained the working title “The Madness Of Professor Lazarus”. Davies asked Greenhorn to provide a substantial role for Martha's family, following up on their introduction in the season premiere, Smith And Jones. The spotlight would particularly fall on Tish, whose full given name was revealed to be Patricia; it would later be changed to Letitia in dubbing. At a late stage, Davies added the material between Francine and the agents of Mr Saxon -- whose first name, Harold, was introduced. This would set up elements to be paid off in the season finale, The Sound Of Drums / Last Of The Time Lords.

“The Madness Of Professor Lazarus” would be the sixth episode of Season Twenty-Nine. It was paired with the third installment, Gridlock, as Block Three of the production schedule; the two stories would bookend the two-part Daleks In Manhattan / Evolution Of The Daleks. The director assigned to Greenhorn's script was Richard Clark, while renowned Doctor Who fan Mark Gatiss was cast as Professor Lazarus. Gatiss had written both 2005's The Unquiet Dead and 2006's The Idiot's Lantern, but he was also a well-regarded actor and had experience of prosthetics from his work with the League Of Gentlemen comedy troupe. This would be valuable, because the production team had discarded the notion of hiring two similar actors to play the older and younger incarnations of Lazarus, preferring instead to transform the thirty-nine-year-old Gatiss into the seventy-six-year-old professor.

A major location in Greenhorn's script was St Paul's Cathedral in the City of London; it could be viewed from Lazarus' office, and was the site of the climactic confrontation with the Lazarus monster. Arrangements were made to film material at St Paul's itself, only for its administrators to back out at a late stage due to concerns about the sequence in which Lazarus fell from the Whispering Gallery. The production team instead approached both the church of St Martin-in-the-Fields in Westminster and the Royal Albert Hall in South Kensington, but neither was ultimately deemed suitable. Instead, the action was relocated to the Cathedral of St Saviour and St Mary Overie -- better known as Southwark Cathedral, given its location in London. This forced a number of amendments to the dialogue, such as the elimination of references to Christopher Wren, the architect of St Paul's.

But while the narrative had been altered to replace St Paul's with Southwark Cathedral, recording would instead take place at Wells Cathedral in Wells, Somerset. Cast and crew spent two days at the location, on October 3rd and 4th. Largely built in the twelfth and thirteenth centuries, it was only slightly older than Southwark Cathedral, most of which was constructed between the thirteenth and fifteenth centuries. Clark's team then shifted to the Senedd, the new home of the National Assembly for Wales in Cardiff, which had been formally opened just seven months earlier. Material in the reception room was filmed there on October 5th and 6th, and from the 9th to the 11th. Part of the latter day was also spent at Churchill House in Cardiff, for Lazarus' rooftop metamorphosis. The final day at the Senedd was October 12th, with the focus now shifting to the action in and around the storeroom, as well as various corridors and the stairwell. The venue on the 13th was Lloyds TSB in Cardiff; it provided Lazarus' office, and the shot of him dying in Southwark Cathedral was captured there as well.

After the weekend, work on “The Madness Of Professor Lazarus” resumed on October 16th, at the Cardiff National Museum. In addition to Lazarus' press conference, Clark shot sequences in the laboratory foyer and the security station, plus the exterior scene in which Francine slapped the Doctor. Filming on the 17th began at Doctor Who's regular studio home in Upper Boat, for footage of the Doctor being hurtled through the air by the explosion. The production then shifted to Cardiff University's Biomedical Science Building, which offered space suitable as Lazarus' laboratory. The remainder of the episode was completed at Upper Boat, starting on October 18th with green screen work and various pick-up shots. Scenes in Martha's flat and within Lazarus' machine were filmed on the 19th. Clark also planned to record on the TARDIS set that day but, in the event, this was deferred until November 7th, when he also completed additional inserts.

The title was not an intentional homage to The Quatermass Experiment

In December, Davies decided to truncate the episode's title to The Lazarus Experiment. It was commonly assumed that the new name paid homage to Nigel Kneale's landmark 1953 science-fiction serial The Quatermass Experiment -- especially given that both Gatiss and David Tennant had co-starred in a live remake of the serial for BBC Four, broadcast during 2005. However, Davies would later insist that this had not been his intention.

The Lazarus Experiment was broadcast on May 5th, with Doctor Who temporarily returning to 7.00pm, where it had been positioned for most of Season Twenty-Eight and for the first two episodes of the current run. This was possible because of a change later in the Saturday schedule: The National Lottery People's Quiz had concluded the week before, and had now been replaced with the shorter The National Lottery Saturday Night Draws. A week's hiatus followed The Lazarus Experiment, with Any Dream Will Do -- usually broadcast immediately after Doctor Who -- taking over its timeslot in order to accommodate the 2007 edition of the Eurovision Song Contest later in the night. This was the first time that Doctor Who had taken a mid-season break since its transmission was interrupted for two weeks over Christmas 1980.

• Doctor Who Magazine Special Edition #17, 22nd November 2007, “Episode 6: The Lazarus Experiment” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #55, 2015, “Story 183: The Lazarus Experiment”, edited by John Ainsworth, Hachette Partworks Ltd.