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

Context articles: Captain America
Captain America: The Winter Soldier
Captain America and the Falcon
Captain America: The Captain
Captain America: The First Avenger
Captain America's shield
Bucky Barnes
The Death of Captain America
Captain America (William Burnside)
Black Widow (Natasha Romanova)

Captain America is a fictional character appearing in American comic books published by Marvel Comics.
 Created by cartoonists Joe Simon and Jack Kirby, the character first appeared in "Captain America Comics" #1 (cover dated March 1941) from Timely Comics, a predecessor of Marvel Comics.
 Captain America was designed as a patriotic supersoldier who often fought the Axis powers of World War II and was Timely Comics' most popular character during the wartime period.
 The popularity of superheroes waned following the war and the "Captain America" comic book was discontinued in 1950, with a short-lived revival in 1953.
 Since Marvel Comics revived the character in 1964, Captain America has remained in publication.
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.
 It is the sequel to 2011's "" and the ninth film in the Marvel Cinematic Universe (MCU).
 The film was directed by Anthony and Joe Russo, with a screenplay by Christopher Markus & Stephen McFeely, who had also written "The First Avenger".
 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.
 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.
Captain America and the Falcon was a comic book series published for fourteen issues in 2004 and 2005 by Marvel Comics.
 The series' title is a reuse of the cover title of Captain America's solo series during a period in which the Falcon was given second billing on the front cover.
 Although never the official title, "Captain America" was cover titled "Captain America and the Falcon" from issue #134 (dated February 1971) to issue #222 (dated June 1978).
Captain America: The Captain is the umbrella title used for the run of "Captain America" from issue 332 (cover dated Aug. 1987) to issue 350 (cover dated Feb. 1989), written by Mark Gruenwald.
 In this story arc, Steve Rogers (the original Captain America) temporarily gives up his superhero identity and is replaced by John Walker, before Rogers decides to return and has to fight Walker for the title.
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.
 It is the fifth film in the Marvel Cinematic Universe.
 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.
 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.
Captain America's shield is a fictional item appearing in American comic books published by Marvel Comics.
 It is the primary defensive and offensive piece of equipment used by the Marvel Comics superhero Captain America; he is seldom seen without it.
 Over the years, Captain America has had the use of several different shields of varying composition and design.
 His original heater shield first appeared in "Captain America Comics" #1 (March 1941), published by Marvel's 1940s predecessor, Timely Comics.
 The circular shield best associated with the character debuted in the next issue, "Captain America Comics" #2.
 Captain America was created by the team of writer-artist Joe Simon and artist Jack Kirby.
James Buchanan "Bucky" Barnes is a fictional superhero that appears in American comic books published by Marvel Comics.
 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".
 The character is brought back from supposed death as the brainwashed assassin Winter Soldier (Russian: Зимний Солдат , translit.
 "Zimnij Soldát"), and later assumed the role of Captain America when Steve Rogers was presumed to be dead.
"The Death of Captain America" is an eighteen-issue Captain America story arc written by Ed Brubaker with art by Steve Epting and published by Marvel Comics.
 The arc first appears in "Captain America" (vol.
 5) #25–42.
 The first issue of the story arc, "Captain America" #25, was the highest selling comic for the month of its release.
 The story arc had wide-sweeping effects throughout the Marvel Universe and was accompanied by the miniseries "".
William Burnside, also known as the Captain America of the 1950s, is a fictional character appearing in American comic books published by Marvel Comics.
 He was created by writer Steve Englehart and artist Sal Buscema in "Captain America" #153–156 (September–December, 1972) as a specifically different Captain America than the Captain America introduced in 1953 in "Young Men" comics.
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.
 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).
 The character was first introduced as a Russian spy, an antagonist of the superhero Iron Man.
 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.