The episode's narrative focuses on an alternative history where the Doctor dies during the events of the 2006 Christmas special "The Runaway Bride".

The episode depicts a dystopia caused by the Doctor's death, leaving Rose to convince Donna to save the world.

Davies' writing and Tate's performance were acclaimed, and the episode was praised for its depiction of dystopia in a scene characterised by the internment of a foreign citizen.

The episode was one of two Doctor Who stories in the fourth series to be nominated for a Hugo Award in the Best Dramatic Presentation, Short Form category.

Donna's decision creates an alternative reality in which she never met the Doctor and he drowns in the flood killing the Racnoss children,[N 2] making him unable to intervene in several other events affecting contemporary Earth.

Changes include the deaths of the Doctor's former companions Martha, Sarah Jane, her son and friends; Jack being transported to the Sontarans' home planet after his team sacrificed their lives; and the space-going Titanic crashing into Buckingham Palace, killing millions of London's residents, irradiating southern England and closing France's borders.

Rose mysteriously appears to Donna, and gives her advice that saves her and her family from the destruction of London, but they are forcibly displaced to Leeds.

[4] The episode's tone phrase was "life during wartime"; Davies reflected his description by comparing the labour camps, to which foreigners such as the Italian Rocco Colasanto (Joseph Long) were sent, to the Nazi concentration camps of World War II – most notably Auschwitz-Birkenau – through script directions and Wilf's expository dialogue:[5][There's an open army truck in the street, 2 soldiers standing by.

[4] Davies and Piper cited her other projects – specifically, her roles as Belle de Jour in Secret Diary of a Call Girl, the eponymous character in the BBC adaptations of Philip Pullman's Sally Lockhart quartet, and Fanny Price in the ITV adaptation of Mansfield Park – to explain that her departure was permanent.

[5] Davies created the expectation of Rose's return by mentioning her in dialogue and featuring Piper in cameo appearances in "Partners in Crime", "The Poison Sky", and "Midnight".

He was several weeks behind schedule and had to decline an appearance at the National Television Awards four days later in order to hand the script in on time.

[7] Davies was delayed due to the death of Howard Attfield, who portrayed Donna's father Geoff, and the difficulty of writing Rose's expository dialogue; he had to rush the script's ending to ensure it was ready to film.

[4] Davies declined to state whether the episode was part of the series finale; he preferred to stay out of the imminent fan debate.

[4] The beetle's normal Earth-like appearance was deliberate; prosthetic designer Niell Gorton thought that familiarity would ease the narrative and cited the cat nuns from "New Earth" and the Judoon from "Smith and Jones" as examples.

[5] The outdoor scenes in Shan Shen – comprising Tennant's entire contribution to the episode – were filmed on 1 December 2007 in Splott and near the Cardiff Royal Infirmary.

The shoot was marred by difficulties: rain delayed re-dressing the alley from the hanzi banners and posters to the Bad Wolf versions; and several extras left at lunchtime because of a misunderstanding over their payment.

Thompson Park was originally scheduled for the shoot; the location was changed at short notice to Sophia Gardens because Tate was suffering from a mild case of influenza.

[5] Because the episode had a low budget, it relied heavily on stock footage and pre-existing graphics: the Titanic's descent into Buckingham Palace and the American television report of the populace being transformed into Adipose utilised footage from "Voyage of the Damned" and "Partners in Crime", respectively, and images of the Racnoss Webstar and the ignited sky had already been created by The Mill.

[4] "Turn Left" was watched by 8.09 million viewers – a 35% share of the total television audience – and received an Appreciation Index score of 88: considered Excellent.

Walker attributed the episode's origin as an allusion to other prominent alternate history works, such as It's a Wonderful Life and Sliding Doors, and applauded the mention of the Trickster as an "unexpected but welcome cross-franchise reference".

[16] He thought that Tate portrayed the "unenlightened" version of Donna far better than in "The Runaway Bride", describing her acting as "far removed from the totally unappealing character she was to start with".

He thought that Billie Piper was "distinctly below par", citing her gaunt and malnourished appearance, new hairstyle, and slight lisp as reasons why her acting was not her finest.

He noted the allusion to the concept of the power of names previously referred to in "The Shakespeare Code", "Last of the Time Lords", and "Silence in the Library", but ultimately theorised that the reason was so Davies could set up the episode's cliffhanger.

He commended Davies for "highlighting the contrasting aspects of human nature" in the aftermath of the disaster: the positive side represented by Wilfred's "Blitz spirit" and the "good humoured" and "morale-boosting" sing-along; and the negative side represented by resentment from the Nobles' new neighbours, Sylvia's depression, and, most notably, the internment of foreign citizens in labour camps.

Walker thought that the country's transformation into a fascist dictatorship was a "veiled political point" written by Davies; he cited "the population of the Daily Mail-reading home counties forced to experience living as refugees and asylum seekers" and UNIT troops aiming at unarmed civilians as reasons why the episode was "the most subversive [the show] has ever been".

[16] Closing, Walker congratulated director Graeme Harper for demonstrating "his incredible versatility" in directing the vastly different "The Unicorn and the Wasp" and "Turn Left", and wrote that any concerns about the plot were "overshadowed" by the script's "inventiveness, intelligence, and sheer boldness".

Comparing it to Sliding Doors and discussing the trope of alternate histories, he thought the concept was overused, but "an intriguing endeavour" and that Davies balanced frivolity and "bleak darkness".

[18] Describing the writing as "powerful ... for a family show ... Wilf poignantly remembers the similar horrors he has experienced in the last World War".

Closing, he wrote that there was a sense that "something was missing from the proceedings", but commented that the episode "serves as a good set up for the two-part climax of season four".