Danny used to be a Network scientist and at some point managed to get out. He then took up the alias of "The Tramp" to stay hidden and to be able to monitor The Network's activity.
In S01E02, although initially in a flashback, The Tramp visits Jack Tate to obtain the unpublished Utopia 2 Manuscript from him. Tate, aware that The Network will be after it, doesn't give it to The Tramp but instead sends it to a dealer in Scotland.
He is found by Jessica Hyde and Ian in a park, his "office", but immediately after she says hello, he punches Ian in the face and runs. Jessica quickly catches him and demands he tell her how he knew Utopia had a part two. After some threatening he tells her that Tate told him years ago and that he wanted it now. He informs Jessica that before Philip Carvel died, he was working on a project called Janus, although he doesn't know what it does. She then forces him to tell her about Milner, an MI5 agent like them.
While Ian goes to find a car for Jessica and himself, she questions The Tramp further and points out that it's odd that Tate wouldn't give him the manuscript. She works out that he is with the Network and strangles him to death.