Daniel "Dan" Hopkins was a UNIT medical officer in the 1980s.
In 1984, (AUDIO: Warlock's Cross) he met and assisted the Fifth Doctor in investigating the Helliax and Annabel Morden. He turned down a request to travel with the Doctor due to the oncoming birth of his child. (AUDIO: The Helliax Rift)
In 1987, (AUDIO: Warlock's Cross) his family died in an accidental house fire, an event which drove him mad with grief. (AUDIO: Hour of the Cybermen) When he returned to duty, he took part in Operation: Warlock's Cross and helped investigate scientists experimenting with alien technology at Warlock's Cross. It was then that Ship made contact with him and began whispering in the back of his mind, wanting to be free. (AUDIO: Warlock's Cross)
A year later, Daniel encountered the Cybermen and became a sleeper agent for them, betraying UNIT in return for making him a Cyberman and removing his emotions so he could no longer feel the pain of loss. Four years after that, the Cybermen began targeting London with their dehydration weapons and the Doctor returned in his sixth incarnation to aid UNIT in investigating the plot. To cover up his betrayal, he killed Lt-Col. Lewis Price and brought the Doctor to the Cyber Leader, although by then the Doctor had suspected him. He was partially converted but the process was halted by the Doctor's tampering with the Cyber-Fleet. In spite of his betrayal, the Doctor saved him, but warned Captain Weaver to place Hopkins in secure containment and to be incredibly vigilant should he ever recover. (AUDIO: Hour of the Cybermen)
By 1994, he was held in the Spar, and recited his multiplication tables to focus his mind. Linda Maxwell broke him out and took him back to Warlock's Cross, where the Seventh Doctor, Elizabeth Klein, and Colonel McKenna followed. Working with Ship to make them doubt their own thoughts, Daniel nearly succeeded in making McKenna destroy Ship, which would've either freed Ship or destroyed it and the Earth, after which Daniel believed the Cybermen would return and save the survivors, ascending to a higher state of being. The Doctor prevented McKenna from destroying Ship, later draining Ship's batteries to allow it to rest, and Daniel remained in the Spar. (AUDIO: Warlock's Cross)