Dennis Brown was a shopping centre security guard. He disrupted the Fourth Doctor's attempt to set off a sequence of events by approaching him when he was preparing to toss a coin and enquiring as to his welfare. Talking to the Doctor, Dennis immediately began categorising him based on his lunacy.
Returning to the original moment, and attempting to trigger the sequence for the second time, the Doctor's efforts were not immediately in vain as Dennis did not approach him. However, Dennis again disrupted the sequence when he interrupted the work of a builder who was winching up a pipe containing the pound coin, per the Doctor's design. The coin fell into a cement mixer.
A third attempt saw Dennis make no appearance at the moment the coin was in the pipe, so it achieved the necessary height to fall into a market awning. Following the success of the Doctor's scheme, Dennis approached him and enquired if he was well. The Doctor told him he was an amazing individual as he was "almost immune to time", being an rare example of an individual who was not fixed in time. (AUDIO: Chain Reaction)