Flajnik and his remaining 16 women will leave his vineyards in Sonoma, California for his hometown of San Francisco in the third episode of the current season, but the arrival of the unidentified female will cause drama among the bachelorettes.
A preview released by the network teases that Flajnik will be left stunned when the woman "storms" the cocktail party to make a surprise declaration, prompting outraged responses from the other women, as well as fainting and tears.
Flajnik will also make an "unusual decision" to end the night, according to the network.
Bachelor host Chris Harrison has teased that the woman's appearance will "trigger a category 5 meltdown" among the bachelorettes, saying: "It was beyond explosive. I didn't think the women would be happy but I didn't think it would be such a huge deal."
He added to Entertainment Tonight: "Someone comes back from Ben's past and to give you a hint, she's from our past as well. It's someone who had a relationship with Ben - at least that's what she said. She called me and said, 'This is the situation. I want to come back'. I said, 'This is where we are. We're in San Francisco. Come join us. It's up to Ben after that'."
This week's episode will also see Flajnik enjoying two more one-on-one dates and another group date, which will see the women donning their bikinis to ski down a San Francisco street especially closed off for them.
Flajnik's individual dates will involve climbing 500 feet to the top of Bay Bridge with a bachelorette who is afraid of heights and enjoying a private concert by Matt Nathanson. ABC teases that another bachelorette will be seen rejecting the offer of a one-on-one date.
Last week's instalment of The Bachelor saw blogger Jenna Burke and financial analyst Shawn Reynolds sent home at the second rose ceremony of the season.
Burke recently told Digital Spy that she views Kacie B, Lindzi C and Nicki as the early frontrunners, describing the trio as her "close friends" on the show.
"I feel like all of them are in good places in their lives. When I got to talk to them and really get to know them they are in happy places and ready for this experience," she said.
"They are genuinely nice people and there is something about them. I really connected with them and watching the show I was thinking I wished I was in their place so I could have had the same experience."
The Bachelor airs Monday, January 16 at 8/7c on ABC.
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