On 8 September 2007, Denny and Nell Mendez planned to attack an animal testing facility with a peroxide bomb. However, the bomb exploded as it was loaded onto a coach on Hesketh Road, resulting in dozens of casualties and a neighbouring shop partially collapsing. As medics arrived to treat the survivors, Denny provided his assistance, and senior charge nurse Charlie Fairhead allowed him to accompany a patient to Holby City Hospital's emergency department, claiming that the walking-wounded were welcome if they were "on [their] side".
At the hospital, Denny consoled Saeed Khan whose daughter Fareeda was killed in the explosion. However, once the department was evacuated following the discovery of a suspicious rucksack, Denny disconnected all of Nell's tubes, citing her beliefs, and revealed their original plan to Saeed and Charlie. Enraged, Saeed threatened to stab Denny with a scalpel, but he eventually relinquished it. Denny decided to kill himself, but Charlie led him to the staff room which was being used as a temporary mortuary. He stated that the victims' families and friends would want justice for their deaths, but Denny was adamant on not going to prison and prepared to slit his neck. However, when F2 Toby De Silva suddenly woke up from a nap, Charlie restrained Denny and urged him to do what was right. They subsequently exited the department, and Denny surrendered to the police. (CAS: "Charlie's Anniversary")