Question: Who does Natasha Romanoff and Captain America join forces with in Captain America: The Winter Soldier?

<P0-S0> Captain America: The Winter Soldier is a 2014 American superhero film based on the Marvel Comics character Captain America, produced by Marvel Studios and distributed by Walt Disney Studios Motion Pictures. <P0-S1>  It is the sequel to 2011's "" and the ninth film in the Marvel Cinematic Universe (MCU). <P0-S2>  The film was directed by Anthony and Joe Russo, with a screenplay by Christopher Markus & Stephen McFeely, who had also written "The First Avenger". <P0-S3>  It stars Chris Evans as Captain America, leading an ensemble cast that includes Scarlett Johansson, Sebastian Stan, Anthony Mackie, Cobie Smulders, Frank Grillo, Emily VanCamp, Hayley Atwell, Robert Redford, and Samuel L. Jackson. <P0-S4>  In "Captain America: The Winter Soldier", Captain America, Black Widow, and Falcon join forces to uncover a conspiracy within S.H.I.E.L.D. while facing a mysterious assassin known as the Winter Soldier. <SEP> <P1-S0> James Buchanan "Bucky" Barnes is a fictional superhero that appears in American comic books published by Marvel Comics. <P1-S1>  Originally introduced as a sidekick to Captain America, the character was created by Joe Simon and Jack Kirby and first appeared in "Captain America Comics" #1 (cover-dated March 1941) (which was published by Marvel's predecessor) as the original and most well-known incarnation of "Bucky". <P1-S2>  The character is brought back from supposed death as the brainwashed assassin Winter Soldier (Russian: Зимний Солдат , translit. <P1-S3>  "Zimnij Soldát"), and later assumed the role of Captain America when Steve Rogers was presumed to be dead. <SEP> <P2-S0> Captain America: The First Avenger is a 2011 American superhero film based on the Marvel Comics character Captain America, produced by Marvel Studios and distributed by Paramount Pictures. <P2-S1>  It is the fifth film in the Marvel Cinematic Universe. <P2-S2>  The film was directed by Joe Johnston, written by Christopher Markus & Stephen McFeely, and stars Chris Evans, Tommy Lee Jones, Hugo Weaving, Hayley Atwell, Sebastian Stan, Dominic Cooper, Neal McDonough, Derek Luke, and Stanley Tucci. <P2-S3>  Set predominantly during World War II, "Captain America: The First Avenger" tells the story of Steve Rogers, a sickly man from Brooklyn who is transformed into super-soldier Captain America and must stop the Red Skull, who intends to use an artifact called the "Tesseract" as an energy-source for world domination. <SEP> <P3-S0> Black Widow (Russian: Чёрная вдова , transliterated "Chyornaya Vdova") (Natalia Alianovna "Natasha" Romanova, Russian: Наталья Альяновна "Наташа" Романова, also known as Natasha Romanoff) is a fictional superhero appearing in American comic books published by Marvel Comics. <P3-S1>  Created by editor and plotter Stan Lee, scripter Don Rico, and artist Don Heck, the character first appeared in "Tales of Suspense" No. 52 (April 1964). <P3-S2>  The character was first introduced as a Russian spy, an antagonist of the superhero Iron Man. <P3-S3>  She later defected to the United States, becoming an agent of the fictional spy agency S.H.I.E.L.D., and a member of the superhero team the Avengers.