Dan Sullivan arrives in Walford — a fictionalized borough in East London — on 7 June 1999, appearing as the latest boyfriend of local café manager Carol Jackson (Lindsey Coulson). They soon move to Walford's local community, Albert Square, where the couple plan to get married after Dan learns that Carol is expecting his child. Their wedding plans surprise Carol's daughter Bianca (Patsy Palmer), as she and Dan had a fling when she was a teenager on holiday many years before. After the pair find themselves secretly reacquainted with each other, Dan and Bianca start an affair — even though she is married to local resident Ricky Butcher (Sid Owen), who ends up befriending Dan. Eventually, Carol discovers the affair and publicly exposes it in front of Ricky and their neighbors. She then disowns Bianca, breaks-up with Dan by chucking him out of her house, and later aborts their child before leaving Walford. Bianca subsequently departs the square after she and Dan are unable to resume their relationship. Shortly afterwards, Dan and Ricky begin to clash after Ricky blames Dan for ruining his life and marriage with Bianca. The hostility between them escalates when Dan begins to flirt with Ricky's sister Janine (Charlie Brooks), which promptly sparks a conflict between the siblings' father Frank (Mike Reid) and Dan himself. One such incident between them involves Frank punching Dan for making remarks on how he brought up his children. This eventually becomes too much for Ricky, who ends up leaving the square in April 2000.