"Deny Thy Father - Part One" is the 512th episode of Casualty and the 18th episode of the 20th series. It is also the first part of the 2005 Christmas crossover between the shows Casualty and Holby City.
Consultants Ric Griffin and Harry Harper interview candidates for a locum general surgery position, including former staff member Nick Jordan. In need of a lift to the staff Christmas party, nurse Kelsey Phillips persuades Nick to take her spare ticket to the event, in exchange for driving her, fellow nurse Donna Jackson, and paramedic Woody Joyner there. Nurse Charlie Fairhead and his new girlfriend Helen are already in attendance at the party, however when Charlie receives a phone call informing him his son has run away, he is forced to head home.
Elsewhere in Holby, taxi driver Bernie Doyle drives his car into a rival taxi company's office. Mitch Campbell, owner of the company, suffers an angina attack, while Bernie injures his wrist. Both men are unhappy that Bernie's daughter, Tara, and Mitch's son, Lee, are dating. Sean Doyle, Bernie's son, locks his sister in his taxi and heads home while Lee gives chase. The car chase causes an accident in a tunnel and Charlie, Nick, Woody, Donna, and Kelsey all end up trapped. Helen suffers a collar bone fracture and is unable to leave the car, so Kelsey sits with her, despite having an intense fear of the dark. Nick tends to Tara, and argues with Charlie over whether they should amputate her leg. Woody and Donna find a man trapped in a car with his infant son. Outside the tunnel Selena Donovan, who was on her way to the party, watches on in horror as the entrance collapses, dashing attempts to rescue those trapped inside.