Jack immediately agrees to meet Martha when she says people are disappearing from the CERN facility in Switzerland, where she's working.
CERN had found a way of examining the building blocks of matter itself; and built the world's largest particle collider which runs from Switzerland to France, in a 27 km round tunnel.
Jack excitedly states that the plan is to collide protons moving at the speed of light together, to produce what would effectively be like the "Big Bang".
He explains that the particle collision is thought to expose parallel dimensions, create black holes or turn the world inside out.
Meeting in Geneva, Martha and the others discuss how they're coping since Toshiko Sato and Dr. Owen Harper's funeral.
Working as a CERN doctor, Julia realised people were falling ill with undiagnosable symptoms.
Martha introduces Ianto as the Welsh Ambassador and Gwen as his wife to the guard, Jack being their assistant.
Arriving in what Martha calls an 'underground city' – where the research takes place – the team view the housing of ATLAS, the largest particle detector (Gwen and Ianto describe it as a jet engine the size of the London Eye), and are met by Professor Katrina Johnson.
Jack suggests they find a link between the patients; Gwen and Ianto head to the control room to hack computers, himself and Martha remaining.
As Gwen and Ianto search for links between the tunnel and the collider, Jack and Martha inform them about Leon.
In the tunnel, the creature poses as Owen, Tosh and Lisa Hallett, tempting Ianto to 'help' them return to life.
[6] The team are heard in pursuit of recurring Torchwood monsters Weevils and reference is made to Martha's fiancé, Thomas Milligan, a character seen in the Doctor Who episode "Last of the Time Lords"[7] and her friend Julia Swales, a character seen in the Doctor Who episode "Smith and Jones".
[8] An alien creature poses as the ghosts of Owen, Toshiko and Ianto's deceased girlfriend, Lisa Hallett, a character seen in the Torchwood episode "Cyberwoman".
The Bekaran deep-tissue scanner first appeared in the Torchwood spin-off novel Another Life[10] and Martha mentions the sonic screwdriver, a device used by the Doctor.
CERN was involved in the production of Lost Souls from its inception; according to writer Joseph Lidster, "they're apparently big Torchwood fans.
[12] He subsequently introduced the theme of grief into the episode, to address the characters' reactions to their friends' deaths.
[13][14] As part of the "Big Bang Day" coverage, James Gillies, director of communications for CERN, wrote an article for Radio 4's website comparing the fictional LHC of Lost Souls with the real LHC: "The CERN of reality bears little resemblance to that of Joseph Lidster's Torchwood script.
"[15] Prior to the special's broadcast, the prospect of a radio version of Torchwood yielded mixed responses from the critics.
Writing in The Sunday Times, Paul Donovan celebrated the special, particularly the contributions of the "beguiling" and "sympathetic" Freema Agyeman; of the special and the rest of the Big Bang Day programming, Donovan said, "This is the sort of output we pay the licence fee for, the sort of ambitious and expensive programming no commercial radio station could ever hope to do in the present ecology of broadcasting.
"[16] However, at The Times, Chris Campling lamented "a special radio episode of the terrible Dr Who spinoff Torchwood, set at CERN and involving the supernatural.
It's as though Radio 4 approached the point of serious educational broadcasting – and then disappeared into the black hole of celebrity.