The episode, set during the last scene of the previous episode "Last of the Time Lords", depicts a humourous encounter between the Doctor's fifth and tenth incarnations, played by Davison and Tennant respectively. "Time Crash" was universally praised by critics who reviewed the episode, and was a ratings success; it was the most-viewed show of the night, and briefly the most-viewed episode of Doctor Who since 2005, with 11 million viewers.
The Doctors realise their TARDISes have merged because the Tenth Doctor forgot to raise his shields. The collision created a supermassive black hole, which the Tenth Doctor counters by creating a supernova. An impressed Fifth Doctor realises that he remembered the solution from watching the Tenth performing it. Before the Fifth Doctor departs, the Tenth Doctor leaves him a heartfelt message:
After the Tenth Doctor says goodbye, the Fifth Doctor reminds him to raise the TARDIS' shields. Seconds after, the Titanic collides with the TARDIS, leading the narrative into "Voyage of the Damned".
Moffat's script started by repeating Martha's departure. The script indicated that "this time, we stay with the Doctor. As before ... the Doctor takes a moment, then slam the controls," and described the Fifth Doctor as having a "frock coat, cricket jersey, and a stick of celery on his lapel". Moffat included several references to Fifth Doctor stories in his script: the Tenth Doctor commented about the Fifth Doctor's attire and his disuse of the sonic screwdriver; and near the end of the episode, he references Tegan Jovanka, Nyssa, the Cybermen stories Earthshock and The Five Doctors, the Mara serials Kinda and Snakedance, the Time Lords' "funny hats" from Arc of Infinity, and the various Ainley Master stories during Davison's tenure.
"Time Crash" was filmed at the show's studios in Upper Boat, Cardiff, as part of the fourth recording block of the fourth series (which included "Partners in Crime") on 7 October 2007. Graeme Harper, director of Davison's final serial The Caves of Androzani, directed the episode. To replicate the Fifth Doctor's attire, the production team borrowed items from the Doctor Who exhibition in Blackpool and knitted a new cricket jumper to reflect the style worn from Castrovalva to Warriors of the Deep. The episode was officially announced by the BBC on 21 October.
The Children in Need telethon was the most-watched television programme of the night, with a final rating of 9.6 million viewers, and figures peaked between 8:15pm and 8:30pm, when "Time Crash" was aired, with a total of 11.0 million viewers. The episode was therefore the most-viewed since the show's revival in 2005, surpassing the revival's premiere, "Rose", which achieved a rating of 10.8 million viewers. Donations also peaked during the episode's airing. When the episode was replayed four hours later, it garnered an audience of 2.5 million viewers. This rating was later beaten by "Voyage of the Damned", which received 13.3 million viewers.
The episode was positively reviewed by critics. Martin Conaghan of TV Squad expressed the belief that the episode was "the highlight of the evening". He mainly complemented Moffat for his writing; he said that Moffat "has a knack for clever paradox-style stories, and managed to capture a fantastic little snippet of emotion, harking back to the early days of Doctor Who," specifically praising the farewell scene. Dek Hogan of Digital Spy mirrored Conaghan's beliefs; he called Moffat's script "witty" and hoped that Davison would return to film a full-length episode. Alasdair Stuart of Firefox News complemented Moffat for his continuing writing quality. He specifically praised Davison's performance and the Tenth Doctor's "you were my Doctor" monologue; he thought the latter was "a lovely way of acknowledging the show's past". He closed his review by calling the episode "Doctor Who at its very best".